In both corn snakes and burmese pythons, providing UVB did show an increase in vitamin D3 in both species, with the difference in the burmese pythons being pretty substantial. There haven't been any studies (to my knowledge) on how that affects the snakes long term, however, just that they can get something out of having an appropriate UVB light available.

I do personally provide my snakes with UVB, and as a result I do see my ball python basking under hers often. I'm not here to argue about it, just to say that there might be benefits to providing snakes with access to UVB, even if they're not immediately obvious to the naked eye.