Here's a common sense reason to leave calcium without D3 in the enclosure (if you're going to leave a supplement) as opposed to leaving calcium with D3 in there. Of course, as we've mentioned above, this is common sense, not researched, and it still may not be what some keepers prefer to do:
It does seem to make sense that nocturnal reptiles don't need that much D3. I don't think they can tell the difference between calcium with D3 and calcium without D3. Consequently, they will be licking the calcium in the tank whenever they feel they need it but will be getting the D3 as well. It makes sense to me to supply the D3 in small amounts separately and use calcium without D3 in the enclosure if desired. Just my opinion.
Aliza
It does seem to make sense that nocturnal reptiles don't need that much D3. I don't think they can tell the difference between calcium with D3 and calcium without D3. Consequently, they will be licking the calcium in the tank whenever they feel they need it but will be getting the D3 as well. It makes sense to me to supply the D3 in small amounts separately and use calcium without D3 in the enclosure if desired. Just my opinion.
Aliza