But I do love how some people love to make assumptions about why people carry spares. "Spray and Pray mentality; poor marksmanship;"
Folks in the gun culture love resorting to cheap, all-or-nothing statements of absolute certainty. The 9mm won't stop anything, 1911's jam, the Beretta's going to blow up in your face...I hear it all day long.
If one has decent training, good situational awareness, the common sense to avoid trouble, and a reliable weapon he's proficient with, he's ahead of 99% of the population. The rest is down to splitting hairs over loadout.
A competent shooter who carries a 15-round automatic with a reload isn't necessarily paranoid, nor does his loadout suggest he'll feel compelled to exhaust all 30 rounds in a confrontation. Nor is someone who carries a smaller gun or array of ammunition tempting fate. It just comes down to comfort level.
I don't feel more vulnerable with a revolver than I do with a high-capacity auto. Yet, if I'm carrying the latter, I'll have it loaded to full capacity. Why? I honestly...well, it seems weird not to. I usually carry a spare mag when I carry one, even though I won't carry it if it's given me trouble. So, why the spare? Don't know. Habit, I suppose.
I know what works for me. That might be different for someone else. To each his own.