Should I Buy all the Same Caliber Weapons?

I run across, gun owners, stating that they choose to buy only 9mm, 40 S&W, or 45 ACP, only weapons, so that they only have to buy one type of ammo.  Being the type of person, that prefers to have everything in order, I can relate with their logic.  But after a few years of gun ownership, I have learned a valuable lesson, when it comes to ammo availability.  During the last gun scare, 9mm ammo was extremely hard to find.  And it was at that time, that I purchased my first 40 Caliber weapon.  40 S&W was easily found on the shelves during this time.  45 ACP was also able to be bought.  The lesson was simple, if you are going to have multiple weapons, buy the weapon in the caliber that makes the best sense.  Example, the Springfield XDS in 9mm is a better choice than the 45 ACP version, because of its small size, and shorter barrel (My Opinion).  Do not be afraid to buy a 380 pistol for your wife.  If she can fire it better, rack the slide better, then don't be afraid of the idea, of stocking two different types of ammo.  She will appreciate the lesser felt recoil, and will be ideally able to stay on target better for follow up shots.  

Now for everyone that subscribes to the SHTF "S_ _ T hits the fan scenarios",  In my opinion, if ammo is limited, and a scenario arrises, where you will need to scavenge for ammo.  Having multiple tools, that gives you diversity, in the ammo you can use, will be paramount.  

Just my two cents on the subject, please feel free to give your opinion, in the comments below.