A spare magazine is a very important accessory to your concealed carry weapon. While having a bunch of additional ammunition is certainly reassuring, the main reason to carry a spare magazine is because they are one of the weak links in the semi-automatic pistol system.
Quite often a bad magazine will cause your otherwise perfectly reliable semi-auto handgun to turn into a malfunction machine. Magazines also have an annoying characteristic of popping off the floor plate right when you don’t want them to and vomiting out your rounds all over the ground. If you don’t have a spare, and the mag in your pistol goes TU, you are stuck with an extremely bulky Derringer. I recommend one spare magazine for double stack pistols and two spares for single stack guns.
Next time you hear “Whutcha need that extra clip fer? You think yer gonna take on a SEAL Team??” you can let them know what they don’t know.
The above is not "The Law as Told By Jay"; it is simply an expression of some opinions that I've formed through my training and experience with several very good instructors. Please feel free to discuss and disagree!