The body does not evolve or fine tune a mechanism to perform one isolated function. We're more efficient than that. (Example: the pathway to the production of cholesterol (via squalene) in the body creates a number of byproducts or metabolites. One is CoQ10, the "vitamin" people are spending lots of money on, and a vital chemical (as a co-enzyme) in the electron transport chain (where we make energy in the form of ATP). Squalene itself is a strong antioxidant. We have less CoQ10 and other important chemicals when we stop manufacturing cholesterol in our cells. In fact, taking statins inhibits their manufacture since it inhibits the manufacture of cholesterol. A number of years ago Merck considered adding CoQ10 to its statin medications to prevent the muscle wasting typical of low CoQ10 levels.
So vitamin D production in our skin is probably part of a larger picture. We know that exposure to light regulates circadian rhythms and production of other hormones - calcitonin, melatonin, as well as cholecalciferol (vitamin D3). All of these have a direct and indirect impact on sleep, lipid metabolism, glucose metabolism, bone mineralization, immunity, and so on.
I think "vitamin D deficiency" could be more accurately described as "sunlight deficiency" or maybe "photon deficiency."
I do wonder about the effects of providing one chemical, vitamin D, the manufacture of which is sunlight dependant, and not all similarly produced chemicals. What impact does a large dose of vitamin D, or a large circulating level of 25(OH)D, have on the manufacture of other light-dependant hormones? How does it affect feedback mechanisms?
It doesn't make sense to supplement with just vitamin D as compensation for sunlight deficiency. If you are going to supplement, why not provide all the chemicals and hormones in short supply as a result of inadequate sun exposure?