I find that I often don't leave feedback because once a game is over, I've lost interest in doing so. I'd have to go to the ended game, and scroll through the messages and my latest moves before being eliminated to figure out what happened in the game.
However, during a good game, I sometimes feel like leaving positive feedback for someone because they made some excellent plays or negative feedback for being unsportsmanlike.
I'm not sure why an end of game restriction is necessary.