Personally, I go out and kill Alliance and/or Horde all the time, depending on my mood, so I'm not seeing a lack of war, but some people seem to dislike the 'status quo' fragile truce between the Horde and the Alliance. Would you guys be interested in rolling Burning Legion? Would it mess the game up too much to have three factions that constantly did war with each other? Would Alterac Valley, WSG, Arathi Basin and EoTS be unplayable if you had to deal with a third team out to kill the other two teams, and killable by both of them as well? Would anyone want a 2x2x2 arena bracket? One thing's for sure: if the Legion became playable, you'd know you were playing the bad guys.
I doubt this would every happen... way too much work to fit it into the established framework. But you may feel differently. So I ask again: would a third playable faction be win or fail? Do you want to see unabashed, unrestrained evil as a playable choice?