Yes, when I carry a semiauto pistol I carry 1 or 2 spare magazines.
I mostly do so because over the course of having worked as a LE firearms instructor for more than 20 years, I've seen at least my fair share of stoppages and other functioning issues that were magazine related, and could only be resolved with the use of another magazine.
When someone has a stoppage which leaves an inconveniently stuck round in the mag feed lips, even after they've ripped it from the pistol ... someone unknowingly depressed their mag catch while sitting or moving about (imagine the reaction as the mag falls out of the pistol when they draw and extend the gun, especially if it falls into shadow, pooled water, falls onto a boot and gets kicked forward, etc) ... a magazine butt/floor plate has been damaged and come loose, and then the magazine releases the rounds out the bottom ... the magazine has become excessively dirty and/or contaminated to the point that rounds won't move freely ... or, an ammunition-related problem in your ONLY magazine suddenly manifests, with the mag load of rounds becoming stuck ... the mag spring picks an
inopportune moment off the range to exhibit weakness, etc.
Sure, having spare ammunition isn't necessarily a bad thing, either. (If it matters, the capacity of my present pistol magazines run 6, 7, 8, 9, 10 & 12 rounds.) I'll typically carry 1 spare, although every once in a while I'll carry 2 spares (old working habit). Just depends.
Oh yeah, I carry spare ammunition when I carry one of my revolvers, too.