I should have mentioned in the OP that I'm talking a smallish single stack pistol, like a M&P Shield. I screwed up and thought of that AFTER I posted and started getting replies.
I agree with you; 15 rounds is surely enough for just about everything that could happen in a non-LEO SD situation. Wish I had that many rounds with the 9mm Shield that I plan to carry most days. But I got tired of carrying my Sig 228, which was my prior carry gun; too heavy, too big for me to carry concealed. Then again, with a spare magazine in my Shield, I'll have 8+1+8=17 rounds.
If you were carrying a single stack pistol, would you carry a spare?