You may think auto-aim is a sweet idea because you underestimate players ability to aim with a controller. I find it to be more challenging and rewarding in the end - thus making the game more fun. As a Killzone (which does not use auto-aim feature at all) player I would at least prefer to have an option to disable auto aim!
Auto aim will stick to an enemy which is not natural at all, and even counterintuitive.
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I totally agree, it seems like Bungie would want to at least add an option since they seem to be trying to draw in the competitive players with their crucible playlists.