A lot of times when doing a dungeon we'll be 3/4 players already in a party, so the random guy will get commendations from the rest of us. Then we'll exit and discover none of us were given one. It's become standard habit now of players not caring to commend or only commending tanks or healers. DPS rarely get commendations even when they perform well and give advice through the dungeon. Even a bad player who fails at mechanics and is aggressively rude towards other party members could just don a tank job and farm commendations just for being a tank.
In all honesty the commendation system might as well not be there. It's heavily biased, it doesn't prove anything and is rarely used properly.