I have two League of Legends accounts: One in which I solo queue with an exclusive champion in order to reach an appropriate ELO for my personal skill level with four random players (to see "how good I am" with that one champion), and a second account in which I play exclusively with friends who aren't as skilled as I am, where I try out champions and builds and gameplay modes that I haven't before. As you can see, one is my "fun" account, and the other is my "professional" account.
I haven't logged into my professional account since Co-op vs. AI came out, which was when I managed to lasso in a number of friends into the game. But just recently I went for a classic, 5v5 Summoner's Rift game on my professional account, and I noticed how different it was. Suddenly, all the "noobs" I had played with were gone. My amazing zoning skills were no longer good enough for my middle-class ranking, and I lost a number of games before settling into an acceptable win-loss pattern.
What happened? Co-op vs. AI happened. The "play for fun" players dropped out of the PvP queue and went to fight robots.This is not to say that the queue was entirely free of leavers and AP Caitlyns, just that over the next few weeks, after about thirty games, I was still matched against players of significantly more skill. I used to get an absolute mish-mash of players more and less skilled than I was. Now my ratio of wins:losses was quickly lowering.
It would seem that I'm the victim in the scenario in which everyone else won in some way. Although the "play for fun" players were not losing more than half of their matches, they could win more by moving to the Co-op vs. AI fights. The "hardcore" competitive players could continue playing their PvP without the scrubs interfering.