First of all, favor items don't work at moment, and if they did, eventually if you play long enough you get the items you want, they're not that expensive (at least not WoW grind for 40 hours expensive). So if you already have the items you want, it's the same scenario.
I admit I've done this myself if we start to get steamrolled, but only when it's hopless enough that playing the last 15 minutes getting horribly beaten all the way along just isn't that exciting. My thoughts at that point are "Well, I could sit here and let's these guys kill me and allies for next 15 minutes, or start another game where I have a chance". My playing time is too valuable to waste on the last 4th of a game where a loss is certain. And really, at that point for the winner, what's 10 minutes of stamrolling a human vs 10 minutes of steamrolling a cpu player? They still get the same favor points, if not more.
Although, I hate it when people do it just out of simple frustration if you just kill them a few times in a row, there's plenty of game to play at that point. Just don't do it too early. Perhaps there can be some penalty to favor points if you quit less than 15 minutes into game or something.
