You run into the old problem that plagues any game with support. Either the support is so powerful you take 1 tank, 1 healer. 1 support, and 2 dps or you just never take a support.
Given wows playerbase is one that is extremely invested in min maxing I can't see how this wont end badly... This is the community that will refuse to use anything that isn't the most optimal version of an item.
Think it is so late to just make the third evoker spec a tank?