The truth is that a lot (most) people have less than optimal vitamin D levels. 400IU is often not adequate. There isn't much of a risk in increasing the RDA for vitamin D. Also, the average person doesn't get 30 minutes of sun exposure every day.
Especially in the winter and in northern latitudes, it's not as efficient. And like you said, I believe the need increases with age.
I agree with everything you said. People come into GNC pretty often and buy vitamin C supplements...they taken in a few grams of vit C a day. It's pretty ridiculous. On the other hand, there are lots of older women that come in and won't buy or use calcium because it's too much of a hassle to take a few capsules each day.....even though their doctors have recommended that they supplement with calcium. They say it's a "quality of life issue".