Obviously auto aim in single player doesn't matter much, since it's just single player and people can use cheats anyways.
However, there is no way Auto Aim will be enabled in GTAO. Do you really think they would make targeting that easy for multiplayer?
Those who rely on auto aim are going to be sorely disappointing when it isn't available online. Alot of people have gotten very comfortable with the 0 skill auto aim option, and it's the reason most people powered through the SP main story. Can't wait to read all the "NO AUTOAIM? GAMES TO HARD" threads on Tuesday.