The main concern with the ammo interchangeability was to have cheaper options for range practice. .38 special is much more affordable than .357 magnum for example. 38 special would be so mild coming out of a lever gun that I also think it would help get the missus into shooting. most of what i have now are all shotguns and rifles. the softest thing i have now is a SKS and that's just because it's a semi-auto lol.
all in all ammo compatibility helps the budget, trigger time, as well as getting a fairly powerful option to use and have fun with.