Proof pressure issues for one, but mainly the ability to use cheap rifled liners with easy-to-make chamber inserts for a .22 Rimfire whereas a full barrel-sleeving job with re-regulation would be needed for a Hornet.

Then there's the cost of ammo for plinking.
...including availability of reloading components (in Australia) for any centrefire in these uncertain times.

And not to be trivialized these days is the legal implication: a rimfire conversion will not change the class of firearm whereas changing it to a centrefire arm is something only a certified/licensed gunsmith can do in most (all?) jurisdictions in Oz.
Of course, technically they will still be "inserts" capable of being removed so in reality no different from the short pistol-caliber insert in a 12-bore for example, but I'd rather not get into an argument with Plod over it.

Anyway, always wanted a hammerless .22 Rimfire SxS double, so its a no-brainer really.

Daryl:
The intention is to use one or more thick collars to support each barrel-liner in the .410 bores, a concentric one flush with each muzzle for aesthetics, and one or two spaced along each liner for support. They could be bored slightly off-centre and rotated to regulate convergence at the muzzle. No doubt this will not be as straight-forward as I am hoping, but should be achievable with some trial-and error.
Point is, I can easily make the chamber inserts and collars on my hobby-lathe, but I would have trouble steadying such a long thin centrefire barrel blank through the headstock.

Damn: now I'm starting to think about moving this project forward in the queue!