Ok, I have some time now, so here are my thoughts and experiences.
TLDR at the bottom
Let me start by stating, because this is going to be a long response, I don't want anyone to feel upset or like they are wrong if I don't agree with their dusting habits

I personally believe that this discussion on with or without D3 is largely a non-issue, but perhaps you can convince me otherwise. I don't think that either side of the advice is wrong, because I think both ways work. I just have my preference.
Firstly, similarly to AHBD, I too have had dragons live long into the 10's and past. My oldest being a 14 year old. However, I have NEVER used only calcium. And I don't even mean that as an exaggeration. In nearly 25 years of owning dragons, I have only used Calcium with D3, and I have always only ever bought and used the RepCal with d3. I've never bought anything else.
My dusting schedule has always been approximately every other day for babies and juvies, and once or twice a week for adults. I rarely if ever dust with a multivitamin, if this is a factor.
My reasoning for the above is from my own experience and what has worked for me, my understanding of how UV exposure works for D3 synthesis after multiple conversations with Dr. Francis Baines, and my potentially anecdotal experience of being on this site for as long as I have.
Firstly, I think a main concern (and really the only reason people say not to use D3) is the concern over vitamin D toxicity. Now to dive into this one, the question must be asked, do we think the potential from D3 toxicity comes from the supplement itself, or from the combination of our stronger UV lamps + the supplement?
If the latter, I would argue that's a non-issue. The body's mechanism for creating D3 from UVB exposure is complex. However, in a similarly complex mechanism, the body can turn off this function. It is well documented that the body will stop producing D3 from UVB exposure once the body reaches whatever set amount it determines is required.
Based on that above established fact, if the bodys D3 needs are being met by supplements, the UVB exposure will not push the body over the line into the toxic levels. It just will stop synthesizing additional D3 from the exposure.
So the next question to ask is whether or not the supplement itself can lead to D3 toxicity. The short answer is yes it could; the real question though is at what point. Does a light dusting 5x a week lead to a toxic D3 level? Does 7 times a week? What about dusting salads and insects every day? Or what if two people follow the same dusting schedule but one lightly coats their insects and another thickly coats the insects?
This is where my time on this site and my anecdotal evidence comes into play. I've been on this site for a long time now. I've seen plenty of dragons suffering from MBD, impaction, neurological disorders, a whole gambit of issues. One thing I've never seen is a dragon suffering from Vitamin D toxicity. Now the caveat to that is vitamin D toxicity likely wouldn't present as the obvious issue as the symptoms are pretty wide reaching and can also be symptoms of a host of other health issues.
So why do I personally think that this site has never seen a dragon suffering from vitamin D toxicity? Because even if the symptoms are pretty generic, one thing this site is really good about doing is going over everything about a dragons care when a dragon comes here sick. If a dragon was being offered too much calcium/d3, as a troubleshooting step, we would have recommended cutting it down. And if the dragon were to have been suffering solely from that, the change in supplementation should have reversed the issue. That has never happened here. And I have never seen a well documented case of a dragon suffering from vitamin D toxicity.
@xp29 Provided his experience with his dusting schedule, and had blood test results to back it up earlier in this thread. One thing to remember though is correlation does not equal causation. And remember, even with great UV bulbs, the body will only utilize the UVB to synthesize D3 if the body needs D3.
In the early stages of MBD, dragons will typically have a high or higher than average blood level concentration of Calcium. This is because the body compensates for the poor calcium absorption by increasing the activity of parathyroid hormone (PTH), which leeches calcium from the bones into the bloodstream. What is more important to pay attention to would be P:CA levels, as this would be a better indication of calcium metabolism, and wouldn't be as effected by factors that can affect Calcium levels in the blood such as protein levels, which do effect blood calcium levels.
TLDR:
There is probably some stuff I'm missing, but the short of it as for why I am for dusting with Calcium with D3 is because I believe the amount of D3 in the supplement is not nearly enough to cause toxicity when following a responsible dusting schedule, and because the body will not produce D3 from UVB exposure unless the body is in need of it. Supplementing with it ensures against relying on potentially improper UV setups to produce the bodys needed vitamin D.
-Brandon