I'll admit that I have quit multiplayer games before, but only if I've been shoved into a match with only minutes to go on a team that's losing terribly and will inevitably lose, or my connection is so bad that I'll constantly freeze temporarily and sudden find myself miles away with people zooming all over the place. I once stayed in a match that was me and one other guy against a team of 8. Neither of us quit, and we got our asses kicked, but we both stuck it out. And then when we completely lost the leader of the other team decided to punish us (this is in Far Cry 3). I couldn't believe what a dick that guy was, but my one team mate was awesome. So it seems that the rest of the team did quit out, which was odd since we were winning.
I've also had cases where at the last 5 seconds almost the entire losing team quit, which is really stupid.
Once in Uncharted 2 I had a connection error, and I was punished by losing one whole level and lots of cash, even when I didn't do anything wrong. Weirdly enough, it's only happened once, so maybe it was a technical error.
So I'm in a bit of a conundrum. Maybe a penalty for quiting towards the last 2 minutes, when on the losing team?