I've never adjusted the settings, so I mostly do

Killzone Shadow Fall on the other hand has it off by default, so I haven't been recently

I don't think Shadow Fall even has an Aim Assist option. I like aim assist in games like Call of Duty but sometimes aim assist makes the game much harder as you will aim at the wrong guy and get killed by the guy you originally wanted to aim at. The aim assist for Last of Us was invaluable though. When you have so few bullets you really don't want to lose any hitting the ground or a wall...

I would have thought not, it would make the MP slightly unfair

The Last of Us's aim assist wasn't too overbearing, you still had to work for your shots and the gunplay still felt hard. As it should, since they were trying to make Joel control clunkily