Mags are a disposable item. Buying spares for when they are scarce or expensive only makes sense. With an eye to any possible SHTF situations where you find yourself using them, having spares only makes sense.
Civil war isn't the only plausible SHTF where you might need more than a few magazines.
I'm not huge on the ZOMG End of the world stuff, but there are plenty of possible scenarios where you could find yourself in a firefight. You could use up a few magazines, lose them, damage them in one shootout.
A Military combat load is 7 mags. (210 rounds )
And in the military we presume prompt resupply, and that you've got assets at your disposal such as the rest of your squad, air support, artillery. If it's just you vs. whoever, you've got no presumption of the calvary arriving anytime soon.
A determined group of attackers might decide that they want what you have. Out of desperation, greed or stupidity.
Drew, you're presuming the only situation where you're in a standoff would be against the military or police.
I personally only keep 4 mags handy for my carbine right now. But I have enough that if something nasty starts developing, I can be equipped to hold out a while. And I will be getting more as time goes on.