You need to be within a specific range of latitudes for the sun to have any effect on your vitamin D levels for the winter months; the atmosphere reflects away the necessary UV wavelengths this time of year. People who have darker skin, like African-Americans, need a much longer time in the sun. For those of us in the Northeast,
Milk is naturally a pretty poor source of vitamin D on its own unless it has been fortified, in which case you'd probably have to drink between 3-4 cups (that's almost a liter, or a quart) per day to meet the DAILY requirement. For those of us who do not drink a lot of milk because it isn't traditionally a part of our diet (I'm Asian) or are lactose intolerant (I'm Asian) that's a pretty tough call. Combine that with the fact it is incredibly difficult to get good-quality, affordable fish on a daily basis in the states (vs. Japan, for instance) and this makes a pretty good case for supplementation. For pregnant mothers, infants, and the elderly especially.
Taking a multivitamin that supplies B-vitamins or vitamin A/E, on the other hand, is pretty silly since these are so widespread in the food supply deficiencies are unknown.
As far as calcium goes milk is a pretty poor source...sure, it is the central source in the Western diet but there are plenty of societies with very low rates of osteoporosis where milk is rarely consumed (once again, Asia). Leafy green vegetables besides spinach generally deliver as much, if not more, calcium than milk does. They also provide a ton of other mineral and micronutrient content, so it is win-win.