There is both some practical and hypothetical advantages to both conversion kits and simple barrel swaps for caliber conversions.
As already mentioned, just saving the money on accessories can be a huge advantage. Between holsters and night-sights, I typically add an additional $200 to every pistol purchase and that doesn’t include magazines (of which I like to have 5-10 spare magazines).
With the continued choke-hold in the .22LR market, the value of the .22 caliber conversions lose their advantage. I still think they are valuable as a training tool and own a half-dozen (CZ, Glock, BHP, Sig, etc.). They work well with new shooters to just learn the basic functions and handling of the handgun along with proper stances before graduating to the centerfire calibers. I used to love using a conversion kit when training transition drills. I only needed a few magazines and just a few rounds to simulate a locked-open slide and then transition to the rifle; much cheaper than regular ammo.
And then, there is just the simple value of having multiple-caliber options in one handgun. Most recall a cycle of ammo shortages and for whatever reason, while one is in short supply others are still available. Most often, it’s between 9mm and .40S&W. Having a Glock 23 with a 9mm barrel gives you some versatility; add a .357 Sig barrel and you really have a lot of options. Honestly, the 9mm KKM barrel resides in the G23 most of the time except when I’m packing in bear country (smaller SE black bears). I already have a couple of G19’s, so plenty of magazines to use.
My current backpacking CCW is a S&W Shield in .40S&W. I like it, but it can wear your hand down after a couple boxes of potent ammo. I go a good mil/LEO discount on a 9mm Shield, and after finding out the 9mm barrel works in the .40 version, that would be a great option between practice and actually carrying. 9mm is often cheaper and much easier to train for some people. I usually buy specialty ammo in .40S&W (like Double Tap’s hard-cast ammo), but I’ll buy 9mm in bulk.
From the hypothetical aspect, if you’re limited to fewer guns for whatever reason, the conversions and barrel swaps can give you a lot of versatility between carrying, training or just plinking. I’ve always like the Glock 23 as the most versatile, off-the-shelf handgun:
.40S&W can give you some pretty heavy bullets for backcountry use.
.357 Sig is a highly effective SD round.
9mm is a very inexpensive training round and provides excellent capacity for SD
.22LR is a great introductory round and inexpensive for plinking.
ROCK6