I always carry at least one spare mag, and not just for the extra rounds. If you bump the release and dump your mag on the floor, it's faster to grab the spare off the belt than to bend down and retrieve the original. And there are other mag-related problems that are best addressed by quick replacement.
"An armed society is a polite society." — Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
— William S. Burroughs