DPS slots should flex from 2 to 4 based on timers for DPS in the normal duty finder dungeons. Then scale the HP of mobs to match.

Reason for the disparity is you don't need as many tanks or healers in endgame as you need DPS. If you look at all the recruiting statics they need DPS and already have their tanks and healers. So players are going from tanks/healers to DPS to compensate.

I know when I queue up for DS with my tank friend, its always DPS we're waiting for.