Loved Lex's reply. For me, streamlining and consolidating calibers is driven by the desire to keep more ammo of relatively few calibers on hand. If I have two pistols that serve different functions in my collection (range practice & plinking vs pocket carry), then if they are both chambered for 9mm, so much the better.
Unfortunately, I (like most people) am not independently wealthy, so I have to make choices about what guns I buy. Consolidating calibers makes sense. I don't stockpile ammo, but I do like to keep enough around to keep me shooting through a shortage. That means I generally like to keep 500 rounds of any pistol ammo I shoot regularly. That gives me a good 3 to 5 range trips before I get low, and I generally replace my reserves after each range outing provided that the caliber is readily available.
Next acquisitions will be a pocket carry 9mm so I can share ammo with my Berretta 92. I would also like to pick up a pistol caliber carbine, the Berretta CX4 Storm, configured to accept Berretta 92 magazines. So, I would have a pocket carry pistol, a full framed pistol, and a carbine that all fired 9mm, and two of those guns would share magazines. Sounds like a dream to me.