I carry a spare because honestly, it's not that difficult to either throw one in a pocket, on your belt, or if you prefer IWB they make IWB magazine holders. You're right, you can't cover every what-if, but if you don't plan on covering SOME what-ifs why do you even carry a firearm? Have you ever had a need for it? If not, why do you carry it? For the "maybe" possibility you'll need it? Why did I need to carry two backup magazines when I worked for a very rural sheriff's department; I rarely had to draw and only a few times had to fire (90% of those were on wounded animals someone hit) - why did I need to carry 46 rounds of ammo on my person and almost 500 in the car?
Why? For the same reason you may have home owners\renters insurance, auto insurance, insurance on your toys, possibly health insurance - how often do you actually use them? You continue to pay for them every month, but they're there in case something happens and you need them. There's way too many variables to plan for every what-if or situation that life can throw at you, that's correct. But, there is no horde of zombies crap.. If you feel safe not carrying a spare mag, then suit yourself. I've never had a handgun magazine malfunction even when using some well used ones, but my life is more important to me than the inconvenience of carrying a spare magazine that weighs less than a pound and takes up about the same room as my cell phone.