Personally I find leavers break the fun experience of a game. But the worst is that I hate join games in progress (backfilling).
Instead of bans, wouldn’t be better to have somekind of leaver queue? For exemple:
Someone leaves x games, next y games they are only queued for replacing leavers.
With that we would not join so often games in progress and leavers would have no way to workaround the penality playing in alternate accounts. Also it seems a more fair punishment according the crime, since leavers would hardly feel some of the consequences of leaving.