Calcium with D3 is not "bad".
It is true that you can overdose them on D3, but the amounts needed would
mean that they would be getting so much calcium that thier entire digestive
system would be blocked solid before that happens.
Now, there are some studies that suggest that beardies cannot really process
the artificially added D3 in the calcium powder (or at least not very well) and
that sufficient UV light is all that's needed.
Some folks go with that opinion, some don't. Frankly, I think the jury is still out.
Although it is proven that Calcium with D3, by itself, isn't enough to prevent MBD.
I've seen several posts from folks who split the difference and dust with Calcium/D3
2-3 times a week and dust with pure Calcium (without D3) on the other 2-3 days of dusting.