If you have an arcane mage, getting The Lightning Capacitor is nice and useful, and up to having the 2/5 tier 5 set bonus combined with a Mystical Skyfire Diamond would make the arcane mage top DPS lists hands-down because Arcane Missiles would proc the meta (have a chance to every hit of it rather).

Upcoming in patch 2.3, Arcane Missiles will not proc the Mystical Skyfire Diamond, so it puts arcane mages back to the backburner for raiding.

arcane missiles cost the entire amount of mana up front and when you have to run out of rain of fire or blizzard or when you have to run away from arcane orb or when you get surrounded by toxic spores, you will have to move interrupting your cast and wasting that large amount of upfront mana you spent. 

during trash sure, great, arcane missile your butt off... but who cares about trash?  bosses are where the damage meters matter and like i already said.  in so many encounters you have to run around out of effects or you become a bomb and have to run out of range of your allies.  so many fights have you on the move and are so melee unfriendly.

there's only one class that can do serious damage while continuously moving.  the same class that never runs out of mana.  the same class that has so much health it dominates in pvp.  the ezmode class with the iwin button on a 2 minute cooldown. 

you want to be top dog on the damage meter?  roll a warlock.