Auto aim to remove any chance of me blaming the game for its failures, because it'll be all on me when I fail a mission. It certainly didn't help in Red Dead Redemption, that game is just ass to me.
If I choose to play with a mouse and keyboard on the PC version, I'd probably play with free aim. Given the platform, head shots just come naturally. If it could auto switch between two control schemes, then great, but I doubt that'll happen.