I just had two back-to-back games with AFKs
The first game, our jungler went AFK about 10 minutes in and did not come back. We lost because of it. The second game, our support went AFK at 15 minutes in and came back literally right before the enemy team took our Nexus. It's fair to say that we lost both of these matches because it was a 4 v 5.
[slayer-jinx-unamused]
I did report both of them, but it seems like nothing ever comes of those reports. I almost feel as though going AFK and screwing your team over should have a higher punishment than just being put into a low-priority queue for a few matches. It's awful knowing that you lost a game just because someone decided to leave because they weren't having fun anymore.