Warlocks don't have a true niche in World of Warcraft. (We're not alone in this.) Like in virtually any RPG that has been influenced by D&D, there are four core classes. Warriors take the hits; Rogues sneak & deal damage up close; Mages deal damage and nasty effects from afar; Priests heal.
Like most RPGs, WoW also has its share of hybrid classes. Paladins fit between a Priest and a Warrior. A Druid fits somewhere between a Warrior, Rogue, and Priest. A Shaman fits somewhere between a Warrior, Mage, and Priest. A Hunter fits in somewhere between a Mage and Warrior.
Naturally, the Paladin, Druid, Shaman, and Hunter can't fill the niche of their "base" classes as well as the base classes themselves. Paladins don't have the combat prowess of Warriors or the healing capability of Priests; they are good in these roles, but not best. The same applies to the Druid, Shaman, and Hunter and the classes they fit between.
To make up for this, they have their novelties and tricks to make them unique; Paladins have the best buffs, Shamans have the best AoE buffs and debuffs in the form of totems, Hunters have the best support in the form of pets, Druids have the novelty of Shapeshifting.
Warlocks are a bit of an odd breed, since we don't cleanly fit between any of the four "base" classes. Instead, we're sort of between a Hunter and a Mage, with a bit of a Priest in there as well. Our pets aren't as good as a Hunter's, and we don't hit as hard as a mage. We certainly don't heal as well as Priests. Our "unique trick" is debuffs and DoTs, which is by no means a bad trick.
But here's where the trouble begins. We're worse at surviving than a hunter and worse at nuking than a mage. This is fine, because a Hunter doesn't hit as hard as a mage or survive as well as a warrior; a Shaman or Druid don't tank, deal damage, or heal as well as their base classes; a Paladin doesn't fight as well as a Warrior or heal as well as a Priest.
HOWEVER, the four other hybrid classes don't have Soul Shards, either. They are sub-par in the roles they imitate. Warlocks are sub-par in many of the roles they imitate, but they also have the annoyance of Soul Shards.
Soul Shards are present for "balance" reasons. I would understand this if Soul Shards made the abilities that required them about as good as the abilities from other classes that they imitate. This is not always the case. On a spell-by-spell basis:
Soulstone:
What it tries to imitate: Rebirth, Reincarnation, Ancestral Spirit, Resurrection, Redemption.
Drawbacks: It has a long cooldown, it must be prepared ahead of time and therefore can't be applied to a dead target.
Advantages: It can be activated in combat, which is shared only by Reincarnation (which has a longer cooldown) and Rebirth (which has the same cooldown). It heals for more than any other resurrection spell, except for Rebirth. Only it and Reincarnation are useful for preventing party wipes. Only it and Reincarnation are good for soloing.
Conclusion: It's a hybrid of Reincarnation (usable on self, usable in combat, wipe prevention) and Rebirth (usable in combat for a large healing upon resurrection). These spells both have cooldowns as long (or longer than) Soulstones, and both require reagents. Thus, the Soul Shard cost may be justified.
Demons:
What it tries to imitate: Hunter Pets.
Drawbacks: Not nearly as powerful as hunter pets. Not as customizable as hunter pets. Summoned slower than hunter pets.
Advantages: More diverse and situationally more powerful than hunter pets (what pets counter spells, remove magic, help stealth detection, seduce humanoids, can be sacrificed into a shield, or provide a stamina buff for the whole party?) Don't need to be kept happy, the skill management is simpler than a Hunter's, you don't have to worry about stabling different creatures, leveling the demons is simpler, reviving demons is no worse than reviving pets.
Conclusion: The raw power of Hunter pets is much better than the raw power of Demons. Demons have some cool tricks of their own, however. Demons require a reagent, but don't require food to be kept happy and you save a lot of world of warcraft gold. Thus, the shard cost may be acceptable.
Ritual of Summoning:
What it tries to imitate: Portals.
Drawbacks: Requires the help of friends.
Advantages: Can bring someone anywhere in the world.
Conclusion: It's different than Portals. Portaling someone to Ironforge isn't useful when you're trying to get a group together for BWL. Likewise, summoning someone to BWL isn't useful when you want training, buying/selling, or the auction house. While different, they can perhaps be seen as "equivalent" in utility. Note that Portals do require reagents; thus, it makes sense perhaps that Ritual of Summoning requires a Soul Shard. However, portals don't require two friends. It seems to me that the two friends should take the place of a reagent, or vice versa.
Healthstone:
What it tries to imitate: Conjure Mana Stone, various healing spells, health potions.
Drawbacks: It costs more mana and heals for less health than any "main" healing spell. It has a 3 minute cooldown, compared to Conjure Mana Stone's 2 minute cooldown and a Healing Potion's 2 minute cooldown. In order to heal someone else, you have to prepare and trade ahead of time.
Advantages: It can be shared, unlike a Mana Stone. The (mana cost/health healed) ratio is better than the Mana Stone's (mana cost/mana healed) ratio. Once it's created and traded, it can be used instantly by the possessor and without screaming for a priest.
Conclusion: The three minute cooldown is pretty limiting. While it has advantages over Mana Stones, its longer cooldown helps balance it out. (As an aside, Mana Stones may be a bit underpowered.) Healing spells have an advantage in every way except for the "be prepared ahead of time" aspect. Why a Soul Shard? Are the other limitations not severe enough?
Spellstone:
What it tries to imitate: Power Word Shield, status fixers.
Drawbacks: Removes beneficial buffs, only absorbs spell damage, sizeable cooldown.
Advantages: A dispel and a shield combined in one, gives a small advantage when trying to crit.
Conclusion: It must be equipped in the offhand, but there are much better things to carry in your offhand than a 1% critical chance increase. It dispels negative and positive buffs, making it rather frustrating. It absorbs less spell damage than a Priest's Power Word:Shield absorbs total damage, and Power Word:Shield has a much shorter cooldown and no reagent. Since it absorbs your buffs, you're left more defenseless against physical attacks. One would think the cooldown is enough for a mediocre ability such as this.
Enslave Demon:
What it tries to imitate: Mind Control.
Drawbacks: Diminishing returns, reduces attack/cast speed by 30%, can't be used on players, Demons aren't as prevalent as humanoids.
Advantages: Demons are powerful compared to many humanoids, lasts longer than Mind Control.
Conclusion: Other than the fact that Demons can be more powerful than some Humanoids, this spell is pretty terrible. Its random-break nature makes the Soul Shard cost painful. If it didn't randomly break and/or have diminishing returns, the Shard cost might be more reasonable. Right now, it is not.
Shadowburn:
What it tries to imitate: Fire Blast, other instant nukes.
Drawbacks: Virtually every instant blast in the game is better in some way, be it more power or added debuffs. Shadowburn requires a talent investment.
Advantages: Warlocks don't have an instant blast spell, other than Death Coil.
Conclusion: The shard cost should really be removed. The cooldown, the weakness of the spell and the eleven-point talent investment really eliminates the need for the shard cost.
Soul Fire:
What it tries to imitate: Any non-instant DD, especially aimed shot or Pyroblast.
Drawbacks: It's the power of one and a half Shadow Bolts for the price of one, with double the cast time of a Shadow Bolt, and a one-minute cooldown.
Advantages: Pretty mana-efficient. And it doesn't require a talent investment, unlike Aimed Shot or Pyroblast.
Conclusion: Aimed Shot has a significantly smaller cooldown. Pyroblast requires a talent investment, but has no reagent, deals more damage, and has the same cooldown. (That being said, Pyroblast is almost certainly underpowered.) The one minute cooldown and the enormous cast time seems to be enough of a drawback to make it also require a soul shard.
Firestone:
What it tries to imitate: Various Shaman weapon enhancements, I guess.
Drawbacks: There's a hell of a lot better things to put in your offhand than something that increases your melee damage and gives your fire spells a slight boost. It's very mediocre.
Advantages: Hard to say. At the highest level, you should almost certainly have something better for the offhand than something that gives you 21 extra fire damage and a melee bonus. And why are you even fighting with a melee weapon?
Conclusion: Garbage. What's the real point of the spell, much less the shard requirement?
Final Thoughts
Note that not a single Affliction spell requires a Soul Shard. This reinforces the fact that we are supposed to excel at DoTs and Debuffs.
Anyway, in the present form, I think the reagent for the Soulstone is justified and the reagent for Demons and Ritual of Summoning is somewhat justified. Every other spell that requires a soul shard has plenty of limitations or weaknesses compared to corresponding spells IN ADDITION TO the shard requirement.
If Soul Shards applied only to Soulstones and Demons, I'd be happy. Furthermore, it'd make sense that you need to give an offering of a soul in order to summon a demon, and it makes some sense that you need to absorb someone's soul in order to store your own. Ritual of Summoning should require either a shard or party members; the shard doesn't make a whole lot of sense, although the party members somewhat do.
Eliminating the Shard requirement for everything but those one to three spells would make more sense, bring more balance to the Warlock class, and eliminate a ton of whining.
Like most RPGs, WoW also has its share of hybrid classes. Paladins fit between a Priest and a Warrior. A Druid fits somewhere between a Warrior, Rogue, and Priest. A Shaman fits somewhere between a Warrior, Mage, and Priest. A Hunter fits in somewhere between a Mage and Warrior.
Naturally, the Paladin, Druid, Shaman, and Hunter can't fill the niche of their "base" classes as well as the base classes themselves. Paladins don't have the combat prowess of Warriors or the healing capability of Priests; they are good in these roles, but not best. The same applies to the Druid, Shaman, and Hunter and the classes they fit between.
To make up for this, they have their novelties and tricks to make them unique; Paladins have the best buffs, Shamans have the best AoE buffs and debuffs in the form of totems, Hunters have the best support in the form of pets, Druids have the novelty of Shapeshifting.
Warlocks are a bit of an odd breed, since we don't cleanly fit between any of the four "base" classes. Instead, we're sort of between a Hunter and a Mage, with a bit of a Priest in there as well. Our pets aren't as good as a Hunter's, and we don't hit as hard as a mage. We certainly don't heal as well as Priests. Our "unique trick" is debuffs and DoTs, which is by no means a bad trick.
But here's where the trouble begins. We're worse at surviving than a hunter and worse at nuking than a mage. This is fine, because a Hunter doesn't hit as hard as a mage or survive as well as a warrior; a Shaman or Druid don't tank, deal damage, or heal as well as their base classes; a Paladin doesn't fight as well as a Warrior or heal as well as a Priest.
HOWEVER, the four other hybrid classes don't have Soul Shards, either. They are sub-par in the roles they imitate. Warlocks are sub-par in many of the roles they imitate, but they also have the annoyance of Soul Shards.
Soul Shards are present for "balance" reasons. I would understand this if Soul Shards made the abilities that required them about as good as the abilities from other classes that they imitate. This is not always the case. On a spell-by-spell basis:
Soulstone:
What it tries to imitate: Rebirth, Reincarnation, Ancestral Spirit, Resurrection, Redemption.
Drawbacks: It has a long cooldown, it must be prepared ahead of time and therefore can't be applied to a dead target.
Advantages: It can be activated in combat, which is shared only by Reincarnation (which has a longer cooldown) and Rebirth (which has the same cooldown). It heals for more than any other resurrection spell, except for Rebirth. Only it and Reincarnation are useful for preventing party wipes. Only it and Reincarnation are good for soloing.
Conclusion: It's a hybrid of Reincarnation (usable on self, usable in combat, wipe prevention) and Rebirth (usable in combat for a large healing upon resurrection). These spells both have cooldowns as long (or longer than) Soulstones, and both require reagents. Thus, the Soul Shard cost may be justified.
Demons:
What it tries to imitate: Hunter Pets.
Drawbacks: Not nearly as powerful as hunter pets. Not as customizable as hunter pets. Summoned slower than hunter pets.
Advantages: More diverse and situationally more powerful than hunter pets (what pets counter spells, remove magic, help stealth detection, seduce humanoids, can be sacrificed into a shield, or provide a stamina buff for the whole party?) Don't need to be kept happy, the skill management is simpler than a Hunter's, you don't have to worry about stabling different creatures, leveling the demons is simpler, reviving demons is no worse than reviving pets.
Conclusion: The raw power of Hunter pets is much better than the raw power of Demons. Demons have some cool tricks of their own, however. Demons require a reagent, but don't require food to be kept happy and you save a lot of world of warcraft gold. Thus, the shard cost may be acceptable.
Ritual of Summoning:
What it tries to imitate: Portals.
Drawbacks: Requires the help of friends.
Advantages: Can bring someone anywhere in the world.
Conclusion: It's different than Portals. Portaling someone to Ironforge isn't useful when you're trying to get a group together for BWL. Likewise, summoning someone to BWL isn't useful when you want training, buying/selling, or the auction house. While different, they can perhaps be seen as "equivalent" in utility. Note that Portals do require reagents; thus, it makes sense perhaps that Ritual of Summoning requires a Soul Shard. However, portals don't require two friends. It seems to me that the two friends should take the place of a reagent, or vice versa.
Healthstone:
What it tries to imitate: Conjure Mana Stone, various healing spells, health potions.
Drawbacks: It costs more mana and heals for less health than any "main" healing spell. It has a 3 minute cooldown, compared to Conjure Mana Stone's 2 minute cooldown and a Healing Potion's 2 minute cooldown. In order to heal someone else, you have to prepare and trade ahead of time.
Advantages: It can be shared, unlike a Mana Stone. The (mana cost/health healed) ratio is better than the Mana Stone's (mana cost/mana healed) ratio. Once it's created and traded, it can be used instantly by the possessor and without screaming for a priest.
Conclusion: The three minute cooldown is pretty limiting. While it has advantages over Mana Stones, its longer cooldown helps balance it out. (As an aside, Mana Stones may be a bit underpowered.) Healing spells have an advantage in every way except for the "be prepared ahead of time" aspect. Why a Soul Shard? Are the other limitations not severe enough?
Spellstone:
What it tries to imitate: Power Word Shield, status fixers.
Drawbacks: Removes beneficial buffs, only absorbs spell damage, sizeable cooldown.
Advantages: A dispel and a shield combined in one, gives a small advantage when trying to crit.
Conclusion: It must be equipped in the offhand, but there are much better things to carry in your offhand than a 1% critical chance increase. It dispels negative and positive buffs, making it rather frustrating. It absorbs less spell damage than a Priest's Power Word:Shield absorbs total damage, and Power Word:Shield has a much shorter cooldown and no reagent. Since it absorbs your buffs, you're left more defenseless against physical attacks. One would think the cooldown is enough for a mediocre ability such as this.
Enslave Demon:
What it tries to imitate: Mind Control.
Drawbacks: Diminishing returns, reduces attack/cast speed by 30%, can't be used on players, Demons aren't as prevalent as humanoids.
Advantages: Demons are powerful compared to many humanoids, lasts longer than Mind Control.
Conclusion: Other than the fact that Demons can be more powerful than some Humanoids, this spell is pretty terrible. Its random-break nature makes the Soul Shard cost painful. If it didn't randomly break and/or have diminishing returns, the Shard cost might be more reasonable. Right now, it is not.
Shadowburn:
What it tries to imitate: Fire Blast, other instant nukes.
Drawbacks: Virtually every instant blast in the game is better in some way, be it more power or added debuffs. Shadowburn requires a talent investment.
Advantages: Warlocks don't have an instant blast spell, other than Death Coil.
Conclusion: The shard cost should really be removed. The cooldown, the weakness of the spell and the eleven-point talent investment really eliminates the need for the shard cost.
Soul Fire:
What it tries to imitate: Any non-instant DD, especially aimed shot or Pyroblast.
Drawbacks: It's the power of one and a half Shadow Bolts for the price of one, with double the cast time of a Shadow Bolt, and a one-minute cooldown.
Advantages: Pretty mana-efficient. And it doesn't require a talent investment, unlike Aimed Shot or Pyroblast.
Conclusion: Aimed Shot has a significantly smaller cooldown. Pyroblast requires a talent investment, but has no reagent, deals more damage, and has the same cooldown. (That being said, Pyroblast is almost certainly underpowered.) The one minute cooldown and the enormous cast time seems to be enough of a drawback to make it also require a soul shard.
Firestone:
What it tries to imitate: Various Shaman weapon enhancements, I guess.
Drawbacks: There's a hell of a lot better things to put in your offhand than something that increases your melee damage and gives your fire spells a slight boost. It's very mediocre.
Advantages: Hard to say. At the highest level, you should almost certainly have something better for the offhand than something that gives you 21 extra fire damage and a melee bonus. And why are you even fighting with a melee weapon?
Conclusion: Garbage. What's the real point of the spell, much less the shard requirement?
Final Thoughts
Note that not a single Affliction spell requires a Soul Shard. This reinforces the fact that we are supposed to excel at DoTs and Debuffs.
Anyway, in the present form, I think the reagent for the Soulstone is justified and the reagent for Demons and Ritual of Summoning is somewhat justified. Every other spell that requires a soul shard has plenty of limitations or weaknesses compared to corresponding spells IN ADDITION TO the shard requirement.
If Soul Shards applied only to Soulstones and Demons, I'd be happy. Furthermore, it'd make sense that you need to give an offering of a soul in order to summon a demon, and it makes some sense that you need to absorb someone's soul in order to store your own. Ritual of Summoning should require either a shard or party members; the shard doesn't make a whole lot of sense, although the party members somewhat do.
Eliminating the Shard requirement for everything but those one to three spells would make more sense, bring more balance to the Warlock class, and eliminate a ton of whining.