In our gamescom demo, presumably using something closer to the original reference design, Kudo Tsunoda expressed reservations that hand and finger tracking would work consistently with the camera simply because human beings come in all different sorts of shapes and sizes. There would be no way to ensure accurate tracking of a child's fingers, for example.
Therefore, for the sake of reliability, the emphasis would shift to tracking the whole body and at that point the need for the VGA depth map was less apparent, although clearly tracking more subtle movements does become more challenging. The lower-resolution depth map also reduces the amount of data being beamed across USB, and decreases processing overhead too.