IMHO the reason for being cautious not to use the D3 too often is that it can build up in the system and cause health issues. I dust at most feedings with a phos.-free calcium powder to help make up for the poor ratio of calcium to phos. found in many of the feeder insects. I dust only twice a month with the phos.-free calcium/D3 powder to ensure that the chameleon gets some D3 without overdoing it and leaving it to produce the rest of the D3 it needs from its exposure to the UVB. I live in Ontario and my chameleons are not outside very often at all.
Background information...I have been keeping chameleons for over 20 years and my veiled females usually live to be over 6 years old and the males even older. I have had many other reptiles (turtles, tortoises, water dragons, a variety of geckos, cordylus lizards, etc.) as well and most of them live long healthy lives and many of them have reproduced too. Many of the reptiles I've kept have been WC's.
BTW...I have kept beardies with the same supplement schedule and they live to be over 11 years as a rule. The same things can happen to beardies that can happen with chameleons if you overdo the D3 from supplements, BTW.
I just lost one of my leopard geckos...she was with me for over 17 years.
Hope this helps!