Well, it was a pain to have to stop shooting video every time one wanted to change aperture. With the ability to use D lenses to change aperture "on the fly" a major problem is solved. Imagine panning from a lighter area to a darker one (or vice versa) without the ability to change aperture to keep the image under control. (One CAN change shutter speed "on the fly" already.)
I am very glad to have found this out, and expect many of those who were unhappy with the "crippled" aperture on video (from D800) will be glad as well.