If you want two guns, that's fine...
... harder to carry and conceal, but not impossible.
A spare magazine is much easier to carry than a second gun, but isn't as fast into action, and although changing magazines is a valid step in clearing a stoppage, it isn't a guaranteed fix.
Either the spare magazine or the spare gun can have its use.
Or there are times when anything beyond the gun and its initial magazine (or cylinder - although this thread was specific to magazines) really is much harder to carry for some reason or another. (Could be a dress function where it's hard to tailor formal wear to hide anything else; could be an undress function where clothing lacks pockets or outer layers; could be any number of things)
My point is and will remain, though, that if your situation and clothing allow the option of a spare magazine without undue aggravation, it's a good idea to carry the spare, partially for ability to use more rounds, and partially for ability to resolve certain stoppages.
Nowhere has anybody said "You must do this." We have said, it isn't really a matter of taste, there are definite benefits to a spare. The question is, when are those benefits worth the extra effort and inconvenience, and when aren't they? That's more subjective.