I don't think the reason for carrying an extra mag or two is so that you simply have more ammunition and that you think you're going to be running your gun dry (though for single stacks, that may still be an issue). The reason people carry extras is that if you have a malfunction in your gun, there's a decent chance you may have to drop your mag. If you're not carrying a spare, you're left with a nice and expensive club.
You may also want to reload after an attack. Like Ramone's story, you don't want to be standing around having just shot (or shot at) someone, and then not have the ability to defend yourself if one of them comes back.
I also think that if you've gone through the difficulty of carrying a gun (permits, holster, belt, daily inconvenience) you really don't loose anything by throwing an extra mag in your back pocket. In a cost benefit analysis, your cost is next to nothing and the benefit could be huge.