field/hunting
I think NewFrontier is on the right track. The mag is a better hunting/field round, a better killer on small game. I'm not sure if the WMR will gain 300-400 fps from revolver length barrels though, comparing equal or similar bullet weights.
I didn't have any use for the .22mag either, but stuck the mag cylinder in my Dad's Single-6 as a novelty and began shooting pests w/ it, mostly armadillo's, a few feral cats, possum, rats, the kind of critters that show up and make a nuisance of themselves around a country place. The mag is a far better killer on that size varmint, and it was apparent right off. The load is the W-W 45 gr JHP.
Mags are expensive, it is not a plinker. And as pointed out, .38's of all description, are a significant step up in power and utility. But, .38 and especially .357 revolvers can be a good bit heavier than a similar .22. My long barreled Ruger Single6 .22mag is significantly lighter than its cousin the .357 Blackhawk w/ the shorter 4-5/8 tube that is in my safe. I've taken to carrying the long barreld .22 mag on scouting jaunts and walks in the woods for opportune type shots on coyote, 'dillos, etc. Fifty rounds weigh nearly moot, but I will never shoot that many on a walk.
The .22mag fills a small niche, but I like it anyway.