While there are no new rewards for receiving commendations, the Challenge Log rewards players for giving commendations, and the Challenge Log resets every single week. So that's something to think about.

I also hit the last commendation achievement, but now I make sure to leave the dungeon immediately after I give my commendation and/or receive the final loot drop (depending on whether I want the loot or not) so that other players are forced to commend other people.