Since it'sa FACT, perhaps you can cite a source?
Some of us take it as a rule of thumb. It's neither a fact nor a rumor nor a law. It's like how I teach people to shoot Tangos and other ne'er-do-wells and perps, when I teach, which is rare these days. I believe it came from Jeff Cooper but I could be wrong: Two to the body, one to the head.
As for Massad Ayoob, a man I respect and admire, but his head is in cop world, not my world:
If you think there is any probability of your getting in a gun fight and needing 18 or more rounds (and you are not military or law-enforcement) you need a serious lifestyle change.
Precisely. When the balloon goes up, or the fecal matter hits the rotating blades, THEN I'll consider burdening myself with an assortment of weaponry.
If there is a downside to carrying a spare mag please explain.
I have enough to carry, so the downside is that it is a bulky item that I don't believe that I need. YMMV. And I did note that there are those occasions when I think a second magazine is prudent, and at those times I dress to permit the carrying of the extra magazine.