My reason for commending DPS may seem strange? Many of us who choose to play dps are not very confident yet in the game mechanics. Some just genuinely enjoy the dps aspects of a game, but for those who lack confidence in themselves, I sometimes commend dps when they've done a good job doing as they should.

...Also, sometimes people leave group so fast you cannot give them a commendation so you have to pick one who is still there who also did well.