I was an RO at a rimfire steel match this past weekend. I could not count the number of stoppages that occurred, they were quite common. Double feeds were particularly common, but I also saw a stove-pipe, full-on extraction failure and a couple of magazines not properly seated situations. Now you can say hey it was rimfire and a competition, but there are lessons here for all of us. Most notably, in many cases in resolving the stoppage the shooter left complete rounds on the table and sometimes failed to have enough rounds left in the gun to hit the stop plate. The competitors who quickly swapped magazines after the initial jam clearing attempt failed were the ones who best recovered and hit all targets including the stop plate.
Carrying a spare magazine (or speedloader) isn’t about having enough rounds to fight off the zombie apocalypse, it primarily about failure recovery. It doesn’t matter if your gun holds 6 or 16 rounds, if the magazine or rounds end up on the ground as part of resolving a stoppage the firearm is useless. Carrying spare ammo is a way to recover.