Spare mags have nothing to do with extra capacity, although it is comforting to know that should ALL my equipment function properly, and I find that after emptying the first mag,I STILL need more ammo, I have another mag to turn to...
I've shot enough IPSC and USPSA competition to know that no matter how meticulous you are with cleaning and maintenance, and no matter how "reliable" your firearm is on the range, when you are moving, shooting and functioning under pressure, the probability that something will fail goes up tremendously. I carry a Para P-14.45 (for both OC and for USPSA matches) and I've NEVER run out of ammo, but I HAVE had to do a few "tap-rack-bang" drills due to faulty ammo, improper grip causing FTF/FTE when shooting from odd positions or on the move, or just plain unexplainable FTFs... Having more mags than you think you'll need makes clearing these failures MUCH faster.
If you have only one mag, and it fails, you are screwed. If you have one or two spares, then you at least have a "fighting chance".
My Para has 13+1 capacity. I always carry one spare mag. Sometimes I carry two spares. When shooting an average USPSA course, I usually have 5 spares on my belt--which is about 3 times the shots needed for an average USPSA stage, but when fractions of seconds count, you don't have time to jack around with clearing a malfunction, and having a metric butt-load of mags does a TREMENDOUS amount for boosting confidence in you rown ability and the reliability of your firearm, because even if something should go majorly wrong with ammo or magazines, having several extras will USUALLY fix the problem.
Again, let me reiterate--we do NOT carry extra mags primarily for the extra ammo capacity the represent. We carry them as a RELIABILITY ENHANCEMENT, to make clearing malfunctions faster, easier, and increase the reliable usage of our firearms if basic malfunctions should occur in their deployment...
I'm not suggesting that you carry a double-stack .45acp with 5 spare mags as your daily OC rig. But 1 spare mag is NOT excessive, and even 2 spare mags is not out of the question, depending on the situation.
Not carrying at least ONE extra mag is pretty much the equivalent of believing in the Keebler Elves--no extra mags means you are living under the delusion that when the SHTF and you have to deploy your firearm in self defense, that EVERYTHING will work with 100% reliability until the problem is resolved.
And THAT is just about as delusional as thinking that placing a 911 call will save your life...