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not sure of what the scenario is where they're picturing themselves needing the second magazine
It's easy to carry a reload, you might as well. Yes, it's a tiny probability that it will help even among the tiny probability that you'll ever actually draw your gun, but it's such a low incremental cost that once you've decided to carry, it would be pointless not to go the extra mile.
I always find this bit the most amusing. I guess you can just about conceive of a scenario where you may need a gun for self defence but I'm not sure of what the scenario is where they're picturing themselves needing the second magazine.