Having the power of a shotshell primer channeled through the ignition path will surely be good for removing moisture and later reducing hang fires. The nipple wear brought about by particulate laden back flow with be eliminated.
Think I'll give it a try, see how it works out. Whether accuracy suffers or not, well, just one way to find out.
Upsides are that it is very much easier to center within the hammer, which is not always the case with replacement nipples of varying length. It is also certainly a hotter flame, and more contained. It can ALSO be used with large rifle primers via the Vari-Flame #209 adapters...which is probably a better priming option than your 209s. It's a lot cleaner to live with that a #11 or musket cap. Weatherproof-ness is improved by somewhat encapsulating the primer, though as you have I'm sure noted, the firing pin just floats in the cap of the Mag-Spark...no water sealing there. If a guy were inventive, I'm sure there's is a neato way to gasket the top of the Mag-Spark...
Negatives are that it is slow-as-sh@# if-and-when you want to reload. If it had some breech lock threads, it would be 90% as strong, but 1/2 the time to use it. I also wasn't ever 100% comfortable with the firing pin sitting out there in the open as much as it does; it's probably nothing, but it bothered me a little. It also kind of halfway seals the bore, and that's not all good. Where the nipple-and-#11 will reset the rifle to half-cock if you are way way too hot on the pressure, the MagSpark will not; hypothetically, this can cause a condition of complete unawareness that you are way past safe pressure for the steel of these barrels because a half-cocked rifle is a GIANT red flag.
I also wish they'd hard anodize the stupid thing matte black.
But yeah. I'd use it with shorter charges of 3F and a Remington or Winchester LR primer in one of these, made by PR Bullet in Canada: