I honestly don't listen much to arguments telling me why I don't need spare magazines. Statistically, I don't
need to carry a firearm at all. Statistically, I don't
need fire extinguishers in the house, either.
John Correia would no doubt laugh at me when I tell him that I always have two spare mags on the hip opposite my primary carry gun, whether that means my 1911...or my Springer XDm, with 15+1 rounds in the gun (I download the mags by one) and 30 rounds on the other hip. Yep; 46 rounds walkin' around. But hey: the weight doesn't bother me so much as the balance. I'd rather carry two 60 lb suitcases than one that's 40 lbs. The Springer usually gets winter-only duty, so in southeast Texas that means a 1911 about 75% of the time. And like others have mentioned, I don't view spare mags so much as extra round count as I do as a failure drill requirement or the ability to quickly reload at a lull in the unfortunate action; been trained to switch out to a fully charged mag whenever practical.
I'm not rich enough to carry two identical guns. Not a fan of cellar-dwelling 1911s, so if I'm ever involved in an incident I'd rather the LEOs confiscate one $1,500 1911 and leave me with another at home, than take both of them. Also, frankly, I would have to retrain myself to carry like a cowboy with one on each hip. As it is, as soon as the "action" stalls my left hand would slap down to my left hip expecting to find a magazine there.
I answered the poll with "Back up gun and spare ammo," but it's never 100% of the time. My BUG is an actual BUG: a pocket carry. Yeah, sort of a mouse gun. But back to statistics, most kinetic gunwork for civilians will be at close distance. If my primary fails, and if a clearance drill and new magazine doesn't solve the problem, it's highly unlikely that I'll then need to be engaging targets at 25 yards with my, er, mouse gun. Most likely, it will be at a distance that may be about the same as using my blade...which I do always carry at least one of (a fixed blade horizontal at the back of my belt if normal untucked Blackhawk or 5.11 shirt over a T-shirt, but always a Kershaw folder).
If I need to dress up, the primary handgun may have to stay behind and the Kershaw get unclipped and just ride inside a pocket. Even with a shoulder holster, it just isn't practical to carry much of anything in a suit and tie; suits aren't cut like they were in the '50s. The BUG is with me at those times, though, and the pocket carry and small size make it easy to lock-up in the console safe if I have to go into a building where I can't carry.
I hate to admit--from a tactics and training perspective--that I vary what and where I carry dependent upon clothes, but it's a mostly unavoidable fact. No primary, the BUG goes into the right pants pocket and a spare mag for it in the left. That's actually the main reason I always carry the BUG: I'm acclimated to having it with me even if dress/location doesn't allow primary carry. So if I don't carry it when I do have the primary, I feel like a situation could happen where I'd expect it to be there...and it wouldn't be.
With the primary and two mags, I'll carry an extra mag for the BUG only if I'm wearing 5.11-style cargo pants. If cargoes, I'll also have an extra folder in the mix and a bigger flashlight. My EDC flash is a Streamlight MicroStream; tiny, can be carried all the time, and will do the job for target identification, but not really a force multiplier in low-light situations.
Also have a 6"-long sturdy, aircraft aluminum tube pepper spray on my keychain. The macho among us might chuckle, but hey. Truth is we may already be at bad breath distance and in the grasp before we can take action, so if a spritz to the eyes can help me quickly clear distance for a better defensive implement, I'll carry it. It's also strong enough to be used as a kubotan if needed.