[user5994461]

This assumes that the game is not already ruined.

Out of 10 games, how many of them are already ruined because 1 or 2 people left, or players are just insulting one another with the most awful thing they can think of. I'd consider quitting to be fair in both cases.

[setr]

in LoL, the system didn't penalize you for a game that already had a quitter; As for the latter issue, you would avoid the penalty by flagging the other players on exit (and it being accepted). I'd assume Dota has something similar in place