Quote from: Blizzard (Source)

I wouldn’t say that this makes it unbalanced. You are free to create a character of any kind so if you feel a different group synergy will work better it will only take you a few minutes to setup. Players are always looking to maximize their usefulness in some way so there will always end up as some class/spec that is the most popular. Expecting a straight even balance across the board is a little unrealistic but we will continually look to balancing things to provide players with the most options possible.