This is a touch of a silly argument as the answer depends on the continuum of incident intensity and the need for redundancy. It's not a yes or no.
Here's my take:
1. Most folk plan for the single mugger scenario.
In this the single mugger accosts you - with enough time to get your gun out. There is little physical contact and probably it ends with no shots fire and deterrence.
Such a situation will be successfully solved with one gun (and even no bullets in it
)
The single scenario, if it progresses to shots fire, usually has the good guy successfully doing a beautiful one shot stop on the BG who flees or is incapacitated.
2. Since intensity of incident is probably a continuum based on:
a. Number of attackers
b. Physical contact
c. Malfunctions
d. Other acts of evil gods to mess with you
You have to calculate if you think the risk will be such that you move away from the modal one mugger - easy to win scenario.
3. I've heard in presentations that most incidents have two more more attackers. Thus, the J frame - 5 rounds is a tad light. I know from
FOF - you miss, you get peripheral hits.
If you are in an incident, like a Columbine, mall shooter - multiple attackers - you burn through rounds really quickly.
Thus, you have to decide if you just carry for the mode - the most common incident or you carry for the some part out in the tail of the incident intensity distribution. It's like Z scores for the stat minded. What is your tail cutoff on incidents you can't handle?
4. Another reason to carry another gun is that if you do plan for a high intensity interaction - malfunctions do occur. Go to a match. Sunday - I saw a Glock throw its Big Dot site into the void and two 1911s decide to do the rack, curse, bang, tap, curse, scream, rack dance. Fighting people rather than paper, you might just ditch it and go for gun 2.
In a FOF, my hand holding a long arm was righteous shot up. Thus, I pulled an airsoft BUG and nailed an attacker with it.
Most of us carry and plan on the relatively low intensity civilian incident. A semi and a spare mag will probably handle most. However, it is not unreasonable to think about the higher intensity incident. We have had civilians in such - two mall shootings and the Tyler courthouse come to mind.
In one of the mall shootings, the officer had only a 1911 with limited rounds and that raised some thoughts in him. In the mall shootings and Tyler, the brave civilians made some serious tactical errors and paid for it.
5. If the altercation goes to H2H, guess what - you can lose your gun. In the recent NTI, I disarmed someone and shot them with their own gun. Said person was a BadGal, so that was OK. Having another gun is not a bad thing.
To conclude, carrying a spare mag or gun is not foolish and not a sign of being some kind of nut. It is a reasoned decision based on where on the incident intensity continuum you want your cutoff.
Setting it at +5 Z scores, for zombies and Red Chinese SHTF with two ARs and 5000 rounds in the car, maybe that's extreme but them zombies are lurking around.
