I think REV only thinks it would be a good idea because he's a PC player (I think?) and he's simply used to hearing that because consoles have a worse form of control than PCs, aim-assist is necessary. Feel free to correct me if I am mistaken though, REV.
I don't really get what you think I was trying to say, but here's my thought again:
I think aim-assist in consoles in general isn't a bad idea. Frankly, it narrows the gap between very skilled and not so skilled players which ultimately means more fun for everyone. However, an aim-assist should never have the head as priority-1-target, because it's the one part of the enemy which will be visible most of the time. Instead it should always target the midriff. That way the recoil doesn't kick the aim up to the head, but the torso and doesn't cause random headshots all too easily.
The aim-assist in GTA IV (and Read Dead Redemption) is very extreme, that is true, but it basicly works the way it should. If you snap onto the enemy, you're pretty much locked on his stomach and struggle to get the crosshairs over his head properly. If competitive shooters had a similar style aim-assist with a lot less "magnetism" going on, they would be pretty handy.
For PC, there obviously shouldn't be any aim-assist what so ever, because a mouse gives you all the power you need already.