Some other results that surprised me from the Private Citizen Equipment Poll is the number of people that are carrying multiple handguns and multiple reloads.
Redundancy vs. capability. We carry a primary firearm to engage deadly force threats at distance, this is a capability we wouldn’t have without a firearm. Adding a backup firearm doesn’t increase this capability, but it adds a level of redundancy that should something happen to the primary handgun (malfunction, disarm, etc.) so that we will still have a deadly force capability at distance.
Magazines add redundancy, but not capability. Having an extra magazine is the same as having a redundant ammunition supply and feeding mechanism for the firearm. The extra magazine doesn’t increase our capabilities, it preserves the deadly force at distance capability under a certain set of conditions: shooting the gun empty, dropping the primary magazine, or a failure of the primary magazine.
Back up guns add redundancy, but not capability. Having an extra firearm has pretty much the same benefits as having a spare magazine and will maintain the deadly force at distance capability in the event that the primary gun should break, or get lost in the scuffle.
Back up gun deployment speed. There is a lot of conventional wisdom that a second gun is the fastest reload or that transitioning to another firearm is faster than clearing a malfunction. A lot of this is dependent upon the operator, the firearms involved, and where they are positioned. Drawing a second gun might be faster than reloading a revolver, but is it faster than reloading a semi-auto assuming some reasonably ergonomic placement of the reload?
Ammunition capacity. There is a balance between how much ammunition we can carry versus the nature of the threat we believe that we will reasonably face. We can invent scenarios where no practical amount of ammunition will be sufficient, but in reality a gunfight that goes to even a single reload is a pretty rare occurrence for private citizens, especially with the prevalence of handguns with more than ten round capacities. Two reloads is almost unheard of in self defense shootings by private citizens.
Extreme circumstances. There are extreme circumstances where simply having a gun and a reload is not going to be enough to solve the whole problem. Putting effective fire on an assailant and/or securing an escape route is a pretty good (if not complete) solution to active shooters or similar situations. Considering that the role of the private citizen is self defense (or protecting a third party) we cannot really expect to stop a Mumbai style attack, but we might be able to mitigate the damage in our immediate area.
Conclusions. Balancing the need for redundancy against all of the other things I need to carry I don’t feel that I have a use for more than one reload on my body. That reload is really not for the extra ammunition as it is for having a redundant feeding system for the gun. I believe that the most likely case for needing a spare magazine would be a fumbled remedial action clearance.
If you believe that you need more than a reload or two as private citizen, I question what other pieces of useful, capability enhancing, equipment are you sacrificing to make room for extra magazines and/or back up guns.