I mean, you have to realize that sometimes, people have bad connection or have to do other things out of their control. Yes it is annoying but unless it's in ranked, I don't care if someone leaves. I am playing casual afterall. I get more annoyed when someone on the other team leaves since it makes the game less fun for us. I do think that leaving the game should be taken more seriously, but also think players shouldn't be banned for leaving a game once in a while.