Because trolls would never abuse such a system. Or even "normal" players who get mad after losing (which most do).
Well, then a way to ban players form your automatch search. I've played against a complete asshole I don't want to play against him again.
Teamkillers, quitters, and afk people should receive some automated temporary penalty though. A mute button isn't too bad either.
I don't see any problem with quitting from a game.
If I see that my teammate is an absolute noob I dont want to spend 40 minutes for a sure lose, I prefer to quit and try again.
Or those teammates than doesn't realize that the game is lost and never want to surrender.
I personally won't play to the game if I'm stuck nearly 1 hour in games that I don't want to play.