I generally don't rage quit at all unless given a good reason. I don't mind taking a loss just cause I still get xp from the match, or if I myself did half decent. But if I'm stuck with the same people that got destroyed the previous game, I'm not sticking around.
Or if I join a game in progress and it's 4 kills from being over and the other team has double, or even triple my team's score, I "rage quit" within seconds.
And sometimes it only takes 1 or 2 people to sabotage all of your efforts. And rather than wait for you to finish inflating your ego and/or K/D ratio, I'd rather roll the dice sooner than later.