I've got a commendation in PvP, and two from HM dungeons. Both when I was DPS... didn't get one on my healer since it was a total accident and I didn't even mean to be in the dungeon on my SCH but I felt bad about dropping out so just went with it... was terrible.
I always give one to people who I think did a good job, but I can imagine people will just click cancel and leave the dungeon ASAP without bothering.