The leaderboards being "hacked out" shouldn't mean it's ok for people to rage quit/leave mid game and cause all other players to lose their round stats for that game.. I gotta agree with the OP it does make me lose the incentive to play when we're doing good then someone times out or goes down and rage quits (although the former is not their fault, but equally as annoying).
If someone leaves and host migration occurs, the round still counts - however the majority of the time I am host and in every game I've played online someone leaves so it NEVER counts. I don't understand why they did this when Black Ops I never had this problem - I guess it was done to punish the rage quitters, something like "well let's cause EVERYONE in that game to lose their stats when someone leaves, that way it makes the rage quitter think twice about leaving because they wouldn't like it done to them" - except it's still an issue that needs sorting... I think the best solution is to either completely derank someone who continuously rage quits, or stop THEIR stats for their next few games, but keep the other players.