See the part of my post where I said that I view it as a "why not" rather than a "must have."
No matter how perfectly reliable your gun has been over the last X thousand rounds, it can still malfunction. It's a machine, and machines (and their parts) break from time to time. As others have said, mags are the most likely failure point of most auto-loading guns.And if you carry a pistol that malfunctions from time to time, why are you carrying it?
There is also the remote possibility that you do get into a truly shitty situation where the amount of ammo that's in the gun isn't enough to solve the problem and you actually need to reload.
Further, if I ever get into the unfortunate situation of having to shoot someone, I kind of want the spare mag so that I can reload and have a full gun while I'm on the phone with 911, in case other trouble decides to find me while I'm waiting for the cops to arrive.