Typically, neither of them spend their whole day (or even half of it) basking. Uromastyx spend a lot of time underground, surfacing only to forage for food, bask for a couple hours, poop, and occasionally mate (for the most part). In the burrows they are safe, they are in a humid environment (because they cannot thrive in an environment that has no area of higher humidity...they will suffer dehydration), and they are comfortable.

Iguanas are similar, in that they do not naturally bask the day away. Typically they are in the trees searching for areas that are not covered by shade, so they only get to come out into direct sunlight a handful of times each day. That's why I personally believe that mercury vapor bulbs are not a good idea, because people do not typically give their pets areas where they can get into some shade. They either have to choose between being warm and in the UV, or being cold and being out of it (if even given that choice). It's just not natural to force them to be under that much UV for that long.

Anyone that thinks reptiles would naturally spend their long days basking is misguided. Reptiles (for the most part) only need to bask a few hours a day to be able to digest their food (and in the wild, to be able to get enough d3 to use the calcium they take in). If they stayed out in the sun all day, they would fall victim to predators, not to mention dehydration.

So with that in mind, UV is not necessary for most (if not all) desert reptiles, due to the fact that in captivity they are given the d3 necessary to use the calcium they take in. I would think it may be a similar situation with iguanas, but I am not familiar with that area, so my comments specifically on that are not very valid. I think that the biggest problem that leads to MBD in iguanas is just that the owners really aren't supplementing enough. Most UV lighting does not give out markable amounts of UVB, and I think you already know my stance on the mercury vapor bulbs

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