A growing body of research suggests that a significant number of people lack vitamin D. Vitamin D deficient is now being linked to a myriad of complications including depression, heart disease, multiple sclerosis, cancer, cognitive decline, decreased immunity, birth defects, etc. The recommend daily allowance is now 1000 IUs, which is up from 400 IUs only a few years ago.
The problem with getting your Vitamin D from the sun is that you would need to expose most of your bare skin for at least 15 minutes to synthesize the required daily dose. Most dermatologists will agree that this is not an acceptable risk to take especially with melanoma on the rise.
If you are interested in finding out if you are deficient in Vitamin D, there are tests available to determine where your Vitamin D levels are. You can also make up for lack of sun exposure by consuming foods that are high in or fortified with Vitamin D such as Salmon or Milk.
The important take away here is that you cannot justify unprotected exposure to the sun because you want to improve your Vitamin D levels. There are a number of other ways to get your required daily allowance.