I only play as SCH and MNK/MCH, and I get commendations pretty often. As SCH I'm doing dps pretty much all the time and shield before pulls, so I guess that's a reason to get commends, maybe. And I'm kinda surprised by how often I get 3 commendations as DD, nice glamours/character or high dps, that's a mistery.

But when it comes to give commendations, I only give them to people who show they are above the average player and also kind or fun to play with.