UVB is not full spectrum light, I have never had to use UVB lighting to grow plants, and in fact doubt using just UVB lighting would be effective at all. You do not need UVB lighting to grow plants in your vivarium.
Again, not claiming UVB is harmful in small doses. But I also do not think it is necessary. Supplementing D3 clearly has some benefit/absorption because from my forum search it appears many people do not use UVB lighting. I think it is more than fair to say that too much UVB lighting is more harmful than too little. Clearly many types of frogs can be raised in an environment without UVB lighting and lead healthy lives.
While treatment for MBD may be exposure to uvb, that does not mean it is necessary in a healthy frog's environment, given they do not possess an unusually high need for UVB lighting. Again, not arguing that UVB in appropriate doses is not beneficial, but I don't think it's necessary with dietary supplementation(again basing this off the knowledge that many people do not use UVB lighting, and have healthy frogs).
I'll make sure to read through the articles when I have time, but a quick skim gives me the impression that the need for UVB lighting on amphibians is still being debated in the scientific community. One the articles stated that no formal studies have been done to assess the needs of UVB and D3 in amphibians.