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.
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
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.