EMP is not that effective since science vessels are gas eating monsters(225 gas) and plus, you need to upgrade EMP shockwave which is 100mineral, 100 gas. you can't make that many science vessels in T v P game. EMP takes 100 mana. <- that's disadvantage. and even if protoss units got hit my EMP shockwave, they can easily retreat their units. and shields can regenerate too. shield battery is pretty cheap if you make them, your units can regenerate their shields fast. P.S.- EMP's range looks really big, but it's not. it's just Blizzard's another trick to scare, and fool people.