There are two primary schools of thought for those of us who have thought about it...
Having a variety of calibers means if there is an emergency, and for some reason a lot of one type of ammo becomes obscure (or only one is produced), you have something in that caliber. So if you have 9, .40, and .45, and the cops are hoarding the .40 and the military the 9, then you can still buy ammo for your .45 and use that.
I am on the other side of the fence. I think that if you would use it for the same thing, you might as well pick one caliber and stick with it. I currently own 9, .40, and .380, but I'm looking at consolidating and shifting to 9mm and .32. They would each have a distinct purpose, 9mm for most of my pistols (used for HD and carry), and .32 ACP for when I want to CC in clothes not designed for it. The advantage here is simplicity.
On the other hand, if a magazine ban does go through, I will amend my plan such that whatever weapon I get has the biggest rounds I can get while still holding 10 (so 9mm in sub-compacts, .40 or .357 in compacts, .45 in duty). However, it will still fit with my philosophy of each caliber serving a purpose, it's just that the purpose would be based on the size of gun.