I really can't believe nobody has mentioned this yet...
But Siege Engines are very often used in the HU mass tower abuse strat. They have high HP, fortified armor, siege damage, can completly pwn air, and worst of all they give the xp of a ghoul?!?! Like wtf?! And actually, they are used all the time actually. HU mass towers with masonry upgrades...like 8-9 towers at his main, 10-13 towers at additional xpos...and then they freely can tech to mass steam tanks & gyros, or mass steam tanks & grifs or knights or whatever as they hide behind towers at any sort of confrontation what so ever...and then the HU just sends wave after wave of tanks to the enemy base taking out zigs, goldmines, town halls, whatever....works best on lost temple, cause of fast xpand and because you can mass towers at xpo so easily. It's basically called the "HU Mass Towers" strat and it's ****ing imba...even Grubby mentioned it in one this one interview cause he lost to it...not to mention Spirit_Moon lost to it during some big tourney on LT vs Rainbow...its ****ing rediculus once HU gets enough towers which makes the base basically untouchable...mix that with mass ****ing tanks which are hard enough to kill, and are taking out all your destros or gargs, or hippos, or whatever air...so you counter the tanks with either siege which die instantly to anything else but the tanks, or melee like ghouls or abombs, which die instantly to the few grifs that are with the tanks...not too hard to adapt for the HU, mass tanks will pwn air, a few grifs will pwn ground...with one or more xpo (which is protected by mass towers), the HU can easily mass whatever....
So ya in general, steam tanks are used in the base race...HU base = mass massonry upgraded, cheap ass, fast building towers....everyone else's base = pwned by take-forever-to-kill-siege-engines....