Well.....
Ive watched the professional matches and when a player drops - EVERYTHING stops until a reconnection is made by the lost player.
Here is a thought - On ranked matches - when a player DC's - pause the game and fill the slot with an equivalent player. Yes PAUSE!
I would expect the matchmaking algorithm stacks 4v4 as even as possible - so when a player drops - odds are you lose on the short team. Also in almost every case - you lose 7+ points. HOWEVER on the other side of that - when you are on the lopsided winning team - averages are +5-6 points.
Either that or maybe consider making the match moot AND penalizing the player who left an ENTIRE level - ex. GOLD 6 to GOLD 5. Yes some players have their game crash - not sure if your logs see that - but that may be a tough one to implement and keep your player base - I believe to a player - they would support the filling the slot and pausing the game. Less so making the match moot.
Just my two cents.
OH additional thought - a player that DCs dont take away points - but ban for 30 minutes - either fix the internet or stop quitting - game crashing....well that is something 343 can address....
Doink