You get no favour if you quit, but seeing as the losing team usually gets only 10 favour, and the winning team can often average over 100, this isn't a big penalty at all.
There are lots of reasons people quit before the game is up. Honestly, the rage-quitters are annoying and immature. Play singleplayer if you cannot live with yourself if you're on the losing end of first blood. For my part, my ISP drops me frequently when I have a P2P application going. Makes it very hard to play demigod 