For all you folks that have the ability to know you'll not need an extra magazine, why not just determine where the trouble will be and avoid it? You don't even need to carry a gun.
If you don't trust yourself and your capabilities, then carry extra magazines. If you trust your capabilities and know that you will not pull out the gun unless you are 100% sure you are going to pull the trigger, and you trust yourself to hit what you aim at, then you don't need the extra magazines.
easyg is so correct. Where do you draw the line. You have approximately a 10X greater chance (Probably 100X, but I'm being polite), of breaking down in your car in the middle of no where, than to need to ever pull a gun on someone. Why not carry 2 spare tires in case one doesn't have air? Or a complete set of hoses, plugs, electronic modules, belts, and all other pieces for your car? You have a greater risk of being hit by lightning, hit by a bus, and definitely being in a car accident or being stranded; than you do of ever having to pull out your gun. Yet, you don't take nearly the same position with those scenarios and you might with a gun.
Mind you, I have nothing against guns. I have probably way too many guns. I have been carrying guns for a very long time. I originally grew up in the New Jersey and New York City area, but out here in wyoming, everyone has a gun. Well actually, it's 86% of all households have guns. We use guns for everything. Hunting, varmints, fun, defense, etc... But even when I'm in Denver, Albuquerque, Salt Lake, etc... I don't have any need for more than the magazine that's in the gun.
Too many people have no idea what the "Real" situation may be like. They imagine. They fantasize of being some sort of Rambo or superhero. But they have no idea. Well, I do have an idea. If I have to pull a gun out and point it towards a person, I WILL PULL THE TRIGGER. There is none of this: "Leave me alone or I'll shoot". Bull Shiite!!! The pistol coming out is the very LAST THING I DO!!! Not the first, or second, or what if.... It's the very last. And if/when it comes out, it's being pointed at someone. And if I still see their face, then I WILL pull the trigger. If I see the back of their head and they're running away, I Won't pull the trigger. And at less than 10 yards, I WILL hit my target. YOU might NOT, and therefor might need extra magazines. I know for a fact that I WILL hit my target inside of 30 feet. Sorry, but know exactly what my capabilities are. Just like I know for a fact that if someone surprises me, and I didn't have a chance to reach for a weapon, and they demand my wallet..... Then guess what? I give them my wallet. Generally, I won't be surprised. I know where I grew up. I know what part of the street to walk. I know where not to walk. I know what to say and what not to say. But when all else fails and I am in a situation where I must pull my weapon, it will NOT be to scare someone, or to threaten them into stopping their advancement, etc... If I pull it out, I WILL shoot them. Twice in the chest, and in 0.00001 seconds, if they haven't dropped, a 3rd shot will be fired. That's what I practice every time I'm at the range. That's what I've practiced for more than 30 years. And I have little to no compassion for criminals. None whatsoever. I have no problem shooting at a person if needed. So definitely have all the magazines you want, or need. I don't need any extras.