I see a big difference between a pistol and a 'backup gun'. For me, my pistol is my secondary and is with me at all times. Should my primary run out of ammo or malfunction, or I need to get into a tight spot I can't with my primary, the secondary comes out.
I bring a 'backup gun' or 2 to games with me in the form of other AEGs in case my primary fails. That way, I have another gun or 2 available. And I'll often also bring along a loaner / rental in case someone else's primary fails and they don't have a backup.