I don't know, I can't think of a least one instance where the sound of a slide being pulled back on a pump shotgun scared a crook intent on doing bodily harm to a friend of mine to simply turn around and leave, whoops I mean rapidly leave. I really can imagine that the sound of taking the Off on a can of mace would scare somebody.
and when the crazies that are out there today, it doesn't make you need to rescue some of them whether or not you cooperate, they're going to kill you anyway. I would much rather die with a gun in my hand, then with my hands in the air, at least I have a chance!