I don't even really have to say that much. The #1 Argument that you shouldn't get banned for leaving/idling a game is:

PEOPLE HAVE LIVES!

We aren't Robots that can just sit down and play all day without having to worry about anything. THINGS HAPPEN IN LIFE! You CANNOT deny it. If whenever you might have to go, You should have a RIGHT to get of the game. Something might happen to a family member that you have to check on in the house, or siblings that you need to check on from time to time, We should have a right to check on them.

The rules concerning bans/level decreases for leaving and idling games is utterly ridiculous.
Another argument against it is also: People can suck at games. If someone on your team or opposing team is just feeding over and over and just not very good at the game, he should have a right to quit without getting warnings of bans or anything. Normally it will HELP if you leave.

I'm sure lots of people would just say "Just stop playing then." But what I hear is "We don't have lives, you do, so stop playing." It ruins the fun out of the game if you can't play it for quitting some games because you have a life.

This is my Legitimate argument over getting banned for leaving or idling games.

TL;DR: People have lives, stop the oppression!