It's certainly a valid point that calcium deficiencies are a bigger problem in growing frogs and it's fair to say that adult frogs who have had the correct exposure while growing up may be a lot healthier having laid down the correct foundation (which could be at least part of the reason that some people are convinced that UVB isn't necessary - they haven't had wide enough experience with raising frogs). However, as Dr Ivan Alfonso explains in this thread it is also important for adults for varying reasons including egg production:
http://www.frogforum.net/showthread.php?t=24225
I would suggest that calcium deficiencies therefore are less noticeable in adult frogs, but they can be deficient nevertheless. In addition though, the benefits of UVB are not of course just limited to the production of D3, the general health and well-being of pretty much every creature on the planet is improved by exposure to sunlight or at least as close as we can artificially re-create it.