For the most part, except for .22's, I think dry firing wont hurt most things, unless youre "really" using them a lot.
In 55+ years of shooting, Ive only ever broke one firing pin, and it was a USGI M14 firing pin in an M1A, that was used in competition and the gun was dry fired in practice constantly as well.
As I said before, I dont think using them can hurt, as long as what youre using stays together, and doesnt leave little bits in the wrong places. Ive personally seen that occur, which is why I prefer properly made/designed factory made snap caps.
That, and they are an added safety measure, as you cant have a live tround in the chamber, if you load the snap cap. I still prefer things that can be instantly identified as such though, over things that look no different than loaded ammo.
If youre making your own, and they are lasting you 20 years of constant use, you may want to go into business for yourself, as you will likely make out real good. I have yet to find any commercially made snap caps that will last much past 6 months of constant, hard use. If yours are holding up that long, you got a winner.