Sold mine off, figure its better to have dedicated. Don't want to break my gun, from use of another caliber. Plus its better to have two and not one.
If you're talking about the Kadet Kit -- and it may be that you're talking about something else -- the "dedicated" Kadet Pistol is the standard 9mm frame with a Kadet Kit mounted on it -- and there's no way to "break" the frame using a .22 conversion kit.
If you're talking about another conversion kit, but a kit on a different type of guns, it's still really no different: the kits do NOT place undue strain on the frame. If anything, shooting the kit on the standard frame (OR related components) will be less wearing on the base gun than shooting the the gun with the standard (9mm, .40) top-end mounted.
It MAY be better FOR YOU to have two (guns), not one -- and only YOU can decide what's best for you -- but for me, I like having a "tuned" trigger that is the SAME with both calibers.