To elaborate on what Tracie said, I think she is mainly referring to supplemental D3 being toxic over time. It is noted that (at least with natural sunlight) once enough D3 has been synthesized via the skin, that the continuing rays actually destroy more D3 from being created. It is an assumption, but if you assume that the same process takes place with our artificial lighting, then it is possible that dietary D3 can lead to overdoses much easier than naturally synthesised D3. Pair that with "if" your dragon is meeting it's d3 requirements via lighting, then it's even easier to understand how supplemental D3 could lead to a problem.
All that being said, I still mainly use calcium with D3. But once i'm out of it I will most likely be switching to only calcium. Especially now that I have a solarmeter and can continue to monitor my UV levels.
In the past (before I got back into dragons) there were no t5's and I definitely didn't have a meter. Calcium + d3 was a good safety measure.
But.. I'm rambling like usual.
-Brandon