While I agree, at least for Vit. D, the issue is what is considered 'deficiency' is quite low by doctors/blood tests. I don't remember the values, but even doctors that a few years ago laughed (yes, laughed) on my wife and mine face because we we concerned about it and the effects of low Vit. D in her pregnancy are now recommending to their patients some supplementation even for known normal levels. (One doctor told us the only concern for low Vit. D levels were for the +65 year olds for example)

But real clinical evidence is still pretty shaky at best. Like 5 years ago there was a lot of hype about Vitamin D and a lot of hope that new studies would conclusively show value in supplementation, but it didn't turn out that way, e.g. http://www.medscape.com/viewarticle/819670

It is also dependent on your profession and location. If you are a collegiate or professional athlete living in the Northern US where sunlight exposure is generally bad for 6 months out of the year, then while your Vit D levels may just be below-average (but nothing major for a normal human), this could be a real issue regarding athletic performance.

Supplementation and testing must always be taken in context. The average sedentary human does not have the same caloric needs as the aspiring professional athlete, nor do they have similar supplementation needs either.

I fully agree. Athletes, pregnancy, high-stress jobs, depression, etc. Each have different concerns and specific nutrition focusing on them can be quite efficient at mitigating symptoms/improving results. Also location as you mentioned and genetics as well can influence the requirements of certain nutrients.

This

I don't remember the exact numbers, but if the sun is out there, around 10min (or maybe up to 30min) under the sun with light clothing will give you the needed Vitamin D dosage.

Now, in high latitude winter, the time needed is in the order of hours. With heavier clothing this becomes much harder.

My parents are general practicioners, and for the last few years they have basically been told that the rule of thumb is to assume everyone is vitamin D deficient until proven otherwise (where "works outside in the sun all day" is an example of proven otherwise).

That doesn't mean that they have evidence that supplementation would actually help people. The currently available data says it could help, could do nothing, and could harm: http://www.medscape.com/viewarticle/819670

I have regularly read that all it takes is 15-30 minutes of direct sun exposure per day to generate the RDA of vitamin D. Is that inaccurate (as someone who slathers his kids with sunscreen I would really like to know)?

Here's a nifty calculator that tells you vitamin D values for exposure times taking season and latitude into account.

http://nadir.nilu.no/~olaeng/fastrt/VitD-ez_quartMED.html

I don't know the answer to that. However, assuming you are correct, I live in a country where direct sun exposure is a rare commodity even in summer, so the recommendation to my parents could still make sense then.