One of the problems that I've run into with my old guild was that when a new game was released, everyone tried rolling healer or tank for easier queues and to 'help the guild', but then we always ended up with to little dps at endgame. You need far more geared dps than tanks or healers for endgame usually, so if you are a healer or tank as main, you dont have as many available slots in a raid group as you would a DPS.

In the end it comes down to, organized groups usually need geared and competent dps, more than tanks/healers, and the DF needs tanks/healers more than dps.