I understand what you are saying, but how many people like actually joining a running game?? With that they take the chance of losing or the game almost being over. So it seems like this would just create a bunch of games from being filled up. Good idea, but not practical.
People hate joining losing games because the loss on their record through no fault of their own.
The solution is for the developers to shift the parameters for a win/loss. Using TDM as an example, if you joined with less than a minute left on the clock or where the score is more than 2K points apart, it shouldn't count on your record - win or loss.
However, your kills and score should always register.
In certain MW2 game modes (S&D or Sabotage, for example), the win or loss wouldn't count if the game had already been going on for a certain time or a few rounds. This wasn't the case for all modes, but should be.