It would probably get abused. Like, if I'm having a terrible game and causing my team to lose, I could just leave to save them.
The fix needs to be to just ban people for some period of time if they AFK more than once every 2 weeks. I'm not saying that family issues are not as important as league, but there is no way of proving you left for a "real" reason as opposed to getting mad and leaving, so there needs to be some kind of punishment.
This should only go into effect, however, after Riot fixes the completely worthless servers/client they have, because I think most of the AFKs come from their end and people shouldn't be punished just because rito wants to make more money so they don't spend it fixing things.