If Weston had shot her, there wouldn't have been a drape hiding her pubic area, but honestly I think that's the detail what makes the picture interesting.
Weston's headless nudes were made that way because it was an integral part of his composition. This emphasized the torso (much like the Venus De Milo) by eliminating the face which is the natural focal point of a picture. Mortensen's composition, OTOH, appears to have cut off the face in the best tradition of porn from that era, to avoid "embarrasing" the model.
Actually, while I like the picture, it certainly reminds me more of a later image by Ruth Berhnard than of earlier ones by Weston.