Agreed. Of course being fully nude laying down at high noon would maximize surface area and require less time in the sun for the same quantity of vitamin D, but having some clothes on is more realistic for most lifestyles and allows one to maintain modesty while getting sunlight and do more activities and be in public. One just needs to be outside for a bit longer 

Rather than setting aside an hour to lay outside half- or fully-naked and tan one's balls and all that, one can get enough sunlight while going in and out of the house doing normal tasks like taking out the trash, going to the market, tending to a garden and animals, exercising, etc