I like pairs, especially for defensive guns as mentioned above, since using one will normally see it confiscated for evidence. I like to keep things simple, especially for the certainly high stress situation that using a gun defensively would entail. I don't want to worry about manipulating a safety or dealing with a change from a da/sa trigger. I only want to have to worry about two things, front sight on the target and finger OFF the trigger unless it's GO time.
It's also fun having a spare second at the range with friends for a little friendly competition. My brother lives several hours away now and had to sell most of his guns when he got maried and things got tight. My best friend lives out of state and lost nearly all his guns to a nasty ex and extended unemployment. When they come in we can still go to the range and have alot of fun shooting the same guns with them using my spares.
Spares IMHO don't have to be exact duplicates. I have a Glock 34 & 17L, the same other than barrel length, and in .454 my Ruger SRH 7.5" & Alaskan are substantially different but I have two weapons that use the same ammo & have the same controls & similar feel. My Ruger Charger .22's however are exact duplicates, both topped with an Eotech and are an absolute blast to shoot against one another.
I personally like limiting calibers so as to only have to stock a couple different kinds of ammo. I mainly have .22lr, 9mm & .45 colt (for the .454's). If my brother brings his shotgun we both use 12ga. Keeps accidents from happening, don't need to worry about dropping a 20ga in the 12, or putting a .44 mag in a .45 colt etc.. Also means if I have a pistol mag it's a 9mm Glock, no worries about mixing in a .40 mag or one for an M&P. Both my .22's can share mags. K.I.S.S. is a very good concept IMHO.