Other than collector value, (the .22 mag Trooper MK 3's were the lowest production of the Mk 3 line) the only benefit is more power out of a rimfire firearm, and the ability to install a .22LR cylinder.
There may have been a very few Mk 3's made with an extra .22LR cylinder, but exchanging those is MUCH more difficult than on a single action pistol, where just pulling the cylinder base pin is all that's needed.
Since the .22 Mag is expensive, and not as powerful as a center fire round, it just doesn't "fit in" and as a consequince isn't a good seller. Years ago, when we found .22 Mag casings at the local range, we used to joke: "Look, some rich fool's been here".
To answer your question: The .22 Mag exists because enough people see SOME use, or need for it. There really doesn't need to be more of a reason than that.
In short, if you are intrigued by the idea of a more powerful rimfire pistol, or want a collectable pistol, buy it. Otherwise I'd look for a .22LR version, which is one of the finest rimfire pistols ever made.
I've owned 5 or 6 of the .22LR's and still have a minty 4 inch model. These are very accurate, and due to the heavy weight, shoot very well, especially in double action.
Currently there are several listed on AuctionArms, GunBroker, and GunsAmerica. The ones on the first two are too high, but the ones on GunsAmerica are affordable.
Hopes this helps.
It's true that .22 mag revolvers aren't as popular as .22LR revolvers. One big reason is that the .22 mag is really more of a rifle round than handgun. Handguns don't have enough barrel length to take advantage of the extra powder in the magnum case so you get a lot of unburned powder and little, if any, ballistic advantage. In many cases, you'll achieve similar muzzle energy with a hyper velocity .22LR as you would with a .22 mag out of a 6" gun. When you combine this with the cost advantage the .22LR has over the .22 mag, the .22LR is the way to go. The only reason I can think of that someone would "need" a .22 mag revolver is if they had a .22 mag rifle and wanted a handgun for plinking/target practice w/o having to take other ammo with.
I have a .22 mag revolver (a 6" S&W 648) but I never shot it. I do however enjoy shooting my Trooper Mk III 6" in .22LR.
My advice: Get the .22 Mag Trooper if that's what you really want but get a .22LR Trooper if you really want to enjoy shooting.
After posting my previous reply to your original post, it got me thinking about a .22 Mag Trooper Mk III.
So I just bought one, NIB!
This one will be strictly a collector piece, the .22LR will continue to see range time.