Mentat, I definitely agree with your synopsis. They may make them grow faster, but that doesn't mean they're healthier. I'm very familiar with the method to grow tarantulas extremely fast (it works with most invertebrates) and it does cause overall weaker animals to be developed.

My opinion is: if you are feeding your animal a proper diet, you should NEVER have to add external supplementation. As in humans, vitamins and minerals that are not taken in from whole foods do little to help your overall health, they just can't be absorbed very well.
I do not give my animals any form of dusting on their foods, and I have never had a problem with MBD, even with ones raised on invertebrate-only diets.