I think Jim was pointing out that there a few sites where you are permitted to dive in a cavern with an OW cert -- the Ballroom at Ginnie Springs being one of them. Why would you want to do that? Well, perhaps if you wanted to see what a cavern was like, but really didn't expect to want to dive in them, and did not immediately see value to you as a diver in a cavern class.
Many, many people do the cenote tours in Mexico who have no intention at all of diving caverns or caves at any other time, and who have no interest in a class, even though such a class might result in improvements in their diving that would carry over into all of their other diving.
Many, many people do the cenote tours in Mexico who have no intention at all of diving caverns or caves at any other time, and who have no interest in a class, even though such a class might result in improvements in their diving that would carry over into all of their other diving.