Personally I have 3 main reason's for giving commendations to DPS:

1. If I see that they have played really well.
2. If they have been talkative / funny / friendly / helpful during the run
3. If they are a new player

Yeah, i do tend to give new players playing dps a commendation as I know how hard it can sometimes be for dps to get it. Also, you don't need that many for the 1st 1-2 level's of commendation achievements... so I consider it a helping hand. However if another dps has been especially good then I might give it to them, or even someone super funny/amusing/entertaining.

Re #2 - I'm sure we've all seen the nonsense that can happen in alliance raids. However I've grouped (either party or alliance) with some really cool dps who have actually helped out ppl failing mechanics, posted quick guides on how to do stuff, etc. They tend to get my commendation if they are in my party... just for being nice and helpful, and not a total jerk to others.