Whether someone else carry's an extra or backup magazine or not is their business. I only can tell you why I do. Each of us must rely on our experience, particularly combat experience, training, weapon selection/limitations, personal abilities & infirmaries, environment, law and PROBABILITIES not possibilities.
We each have our own "STEW" as I like to call it. We make it and we must eat/live with it, or not.
I apply my own extensive combat experience, 60 years of shooting experience, 56 years of competition & large/small game hunting, 48 years of my training, 43 years of instructing experience both rifle/carbine & handguns. Add to that that I have always been a shooting/gun enthusiast, I have my own way of doing things.
I have always carried at least one reload for any weapon I carried, rifle/carbine or handgun. Although I carried a 1911 in the Marine Corps and most of my two 13 month tours in Vietnam (3rdMARDIV).
Most of my life out of the Corps I carried a revolver with, wait for it, one reload 6 rounds. When I was transitioning to bottom feeders, in the late 80's early 90's, I continue to carry one reload, whether the weapon was a SIG 220/8' 226/15 228/13 225/8, 1911/7' Glock 17 etc..... Today I most often CCW/EDC A Walther PPQ classic/m1, in a Milt Sparks VM II, with one additional magazine in the Bill Wilson mag/flashlight carrier on my belt. I have one for my 1911,s too.
In my world STUFF still happens, but for my situations I feel that carrying an extra magazine AND light, is what I like for me. No doubt your choices are what are best for you for your reasons. Smarter guys than me do it differently.
Good luck, and may you never have to learn if your way really does work for you.
Fred