manta49, if you've been shooting long enough, you'll know by now that any gun can have a stoppage... and Murphy's Law says that will occur at a bad time.
I've had revolvers, autos, and a shotgun all hiccup on occasion. I don't believe in a perfect machine. Even the airplanes I fly have a lot of design redundancy, because things break.
So, if one thinks one should have a gun, it's not unreasonable for one to think a spare isn't a bad idea.
How many single engine airliners do you see out there?
Edit: I don't know whether your claims of your police and military only carrying individual weapons are accurate; I suspect, though, that in those parts of the UK where the police and soldiers go about armed, they do not do so alone. Even with one gun a piece, they have some redundancy.