I've never needed to use my jumper cables but I have them any way. Same for car insurance and my CCW and all sorts of other things are for the "just in case" scenarios.
I dont think anyone is arguing that bear spray won't do anything. If you have a disinterested bear that is only looking to poke around then it will probably encourage them to look elsewhere. However its the big snarling pissed off, hungry beast of an animal that most of us are worried about.
I know what the human body is capable of, and I know that animals, bears especially, make us look pathetic on the physical side of things, so I'm not convinced that some goop in a can is going to save my hide when mother nature's furry spawn is charging.
Now if you can guarantee me that every bear that I will ever run into will be nice and polite, then I'd give bear spray a chance. If you could guarantee me that it wouldn't be a windy day, that I would have enough composure to sit still, wait for the bear to be in range, hit it in the face while its doing upwards of 30 miles and hour, and that in the 10 or so yards that I have before I'm an appetizer it works its magic and totally incapacitates the bear, then I'm sold.
Now let me introduce to you a good friend of mine. His name is Murphy...