Depends a lot of the situation.

Tank: If can take the agrro and manage the CDs
DPS: If the mobs/bosses die fast. But it is hard to know which DPS do that sometimes.
Healer: If can maintan me alive in extreme situations. I don't care if DPS or not.

I only take into account DPS on DPS classes. I dont' mind tanks of Healer have low (or 0) DPS. Honestly, sometimes I discard giving commend to a healer if he abuse on DPS, letting me almost die (I wont' give him never if I die for that of course).