The title pretty much says it all. You see all the "purists" constantly trying to shut people down that want class balance by saying "just go play Burning Crusade when they release it". What they really mean to say is that they don't want the competition for loot at end game raids and in PvP so they want things exactly where it was with Vanilla. To that line they use I say that it wont make a difference if a Spriest or Boomkin is doing the DPS compared to Mage or Warlock and the same thing goes for Ret or Kitty compared to Rogue or Warrior.
Then they bring up the excuse of "it will make hybrids to OP in PvP". I think Blizzard is smart enough to know which spells/abilities are used in PvP compared to PvE and buff the ones that don't really cross over well so that this wont happen and if it does oh well. Who else here would like to see Rogues get owned over and over, Rets destroy a warrior, or mages/warlocks get blown up by a hybrid caster in PvP because I would die laughing if that would happen. Again, certain "purists" already have a plan of what they want to do and dominate in PvP with the old way things were and don't want that being messed with so I wouldn't shed a tear if their plans got ruined.
Lets look at it this way. This is just guestimates but lets say just 25% of specs are used at endgame content. A fraction of that 25% are purists that would leave if you balance classes and they actually have competition for loot. Who cares if they leave because now we just gained a ton and I mean a ton of players by letting people actually play the specs they want in end game and pvp. The amount of players that you would gain by letting them play their favorite specs would eclipse the amount of "purists" that would leave. Blizzard is all about making money and if all they have to do is do a quick number tweek on a spell that isn't used much or doesn't cross over from PvE to PvP very well then I don't see anything stopping them from doing that.