Well in your (OP) example, there was a team of six players- IF this is the case and all of those players are average or better then the matchmaking of any match they are in is most likely off. So I agree with guys leaving for this reason.
I play regularly with 12 different guys but we limit our party to 3 guys for two reasons...
One- we're all 2+ kdr players so trying to find a lobby that can balance more than 3 of us is improbable.
Second- when you play with more than 3-4 players I guarantee one of you is gonna be laggy as hell which just makes it not even worth it.
There is also the idea that sticking in a match that is 3v6 for example is somehow a good thing.. but in what other competition do any of us play with an uneven number of players? For example do you continue to play a basketball game if it's 3-5? No, someone would switch- Or 4-5, someone would sit.
So I know we all want good matches where all players stay until the end, however if there is no way to insure fair matches (like with league play), then you shouldn't penalize someone for not having fun. It is just a game..
The best defense against teammates quitting is to find friends/a clan to play with.
Personally, I would rather have a guy quit halfway through the match if he is going 2-12 as he is just feeding streaks.
When there is only one or two opponents left (providing they have a little skill), even against a decent team of 6, they can do OK as the full team can start getting bored and making mistakes, so it can actually work out in your favor.
As far as getting put into losing games, well not too many quit midmatch when they are stomping the opposing team, so faaar less often you are going to join a winning side.
I don't rage quit on this game like I did in all Call of Duty's in the past...I still leave games though, a lot. I only leave games on three occasions. 1 - Spawn killed twice in a row, 2 - Invites from friends, 3 - Getting thrown in to a game either halfway over or a game where the enemy is about to win.
Personally if I was to penalize a player/clan it would go something like this...
 While playing solo: A three strike rule: Quit three time in a 24 hour period results in a 24 hour ban.
 While two or three members' playing in the same squad/team and have the same clan tag: the whole clan gets demoted 1 rank after being warned.
Yes this maybe controversial, so think before entering a lobby no matter how it's going for the sake off "good sportsmanship" just stick it out after all it's a game.
Talking about good sportsmanship and you want to ban players for 24 hours!? Why don't we charge their credit cards and kill their first born children whlie we are at it too! LOLZ! Let people back out, why on earth is this so important to people? I got a little frustrated a week ago when I was playing cranked and the enemy team left 1 player on the team and you can't play cranked against one player! I don't mind that they left, I was just curious as to why more players didn't get put in to the game...for the last 7 minutes of the game!
I forget what release, maybe MW3....but it only put me in fresh servers that were in the limbo menu about to start. I don't think I ever joined an already started game. Not sure how this was done, but it was awesome!
I've been put in my fair share of already losing teams...which sucks....but I've also had my share of being put in a winning game...so I take good with bad. I also care less about "stats" (although I still try to play at my best), so my opinion may not be viable to others.
"...I got a little frustrated a week ago when I was playing cranked and the enemy team left 1 player on the team and you can't play cranked against one player! I don't mind that they left, I was just curious as to why more players didn't get put in to the game...for the last 7 minutes of the game!..."
You got a valid point imo.