As a DPS I find I don't get many commendations, even when I am clearly carrying a bad DPS or casting sleeps to help out with low gear tanks / healers. When I am giving mine out I tend to favor the other DPS in group just because I know they probably don't get any either, if they were bad and I carried them I just give it to tank / healer.

The commendations I do have came from groups where we agreed to circle vote so everybody gets one.