As jestermeister said it can affect the balance of a map quickly. People have been quoting halo matches for years and I'm sure there will be some form of punishment for quitting games.
As far as joining all I want is the option to not join a game in session. I would hate to constantly coming into matches on a losing team. The reason it would probably be the losing team is that of is probably the reason the original player left in the first place. I'm ok with people joining my game as long ad I don't have to join their's.