It's probably too early to call this system useless.
If the bonuses from commendations are worthwhile, people will start to remember and vote each time they complete a dungeon.
Right now people just forget and leave without voting. Creatures of habit most likely.
I'm wondering if you are able to give commendations if the final boss is not defeated and time expires. Sometimes these situations really spotlight the player willing to go above and beyond with helping new people "learn the ropes".