Because DPS is hard to gauge outside of things like Goad, stuns, and mechanics (all things I'll commend for) they usually get my comm if the tank or healer were dropping the ball.

DJ Tank spinning mobs like records? Melee DPS comm for tolerating that.
Healer doesn't have any spells besides Cure on their bars? DPS comm.

Usually I just comm whoever I feel made the run go smoothest, so the above usually aligns with that.