In my first post in this thread, I didn't mention that I served as a LE firearms trainer for 26 years, and during that time I also taught some private citizen classes for approx 10 years. Out of all those years of working on live-fire training and qual ranges I came to the conclusion that when you work enough training ranges for enough years, a significant number of the firing line "problems" you can observe are most easily and quickly resolved with the availability of another magazine. Doesn't matter if it's a single stack or a double stack gun/mag combo.
The magazine is at the very heart of the ability to run a semiauto pistol (that uses detachable magazines). Magazines are assemblies of parts. Primary magazines can sometimes be susceptible to damage, wear or contamination to the extent that function may be compromised. If you've never seen someone's primary magazine suddenly disassemble itself on the firing line (think unrecognized floorplate damage that picks an inopportune moment to make itself known), you're fortunate.
Belt holstered guns that experience being bumped into doorways, chair arms, veh door pillars and other normal objects can often have the point-of-contact be the butt of the magazine.
Then, there's the potential for mags frequently dropped onto hard surfaces (concrete range surfaces) to cause cumulative damage that might compromise the floorplate securely remaining on the mag, under the heavy spring pressure of a full mag load.
If you've never experienced coming back to your veh and seeing your primary pistol mag sitting on the seat, because you didn't realize your holster - when you were seated in the veh - had somehow managed to allow the mag catch to unintentionally be depressed, and the mag to pop free of your gun when you rose from the seat ... you're fortunate. Like those extended, more easily activated mag catch buttons? Consider making it easier to depress the mag catch might not just be something that happens when it's your thumb reaching for the mag catch.
If you have a second mag, you may have enough opportunity and time to address a problem involving the primary mag.
Lawfully carrying a handgun as a dedicated defensive weapon is a study in achieving a balanced compromise that suits your perceived needs. Hopefully, your perception of what you anticipate needing works out to be accurate and appropriate to address whatever you encounter. If you choose to intentionally limit your options to the use of only having 1 mag available, at least keep in mind the consequences, and try to have a Plan B.
FWIW, I often do that myself when deciding to carry one of my LCP's. I carry one of them when I assess the potential threat and risk level to be such that I don't really expect to even need to be armed ... (and one of my .38/.357 snubs won't fit in a jeans/slacks pocket that will accommodate the LCP). The very short and small/thin grip frame of the LCP, and the slide not locking open on the last round, makes reloading/chambering the LCP a study in focused, fine manipulation. While I sometimes carry a spare LCP magazine, it's not a frequent practice. Why? Because I figure that if I exhaust the small capacity of the LCP with its primary mag, and I haven't resolved the immediate problem, I may not have the time left to me to do the juggling and manipulation to load that diminutive second mag. Plan B.