This was a must have as horde teams grossly out number aliance ones the queues to fight an alliance team were getting longer and longer.Also, across the board horde teams were outstripping alliance ones, the top rated bg teams have a very high horde concentration. This was the only way forward.
Without this, you'd have mass outcry from alliance players that horde were OP'd, you'd get mass cry from horde players that matches took far too long, and yet another potentially cool feature would be mediocre at best. In blizzards eyes, compromising the faction integrity of the BGs is a small price to pay.
NExt stop, - who knows they may even allow you to choose your faction instead of it being fixed by race.