Well, I think most treknophiles explain the undercut as an aerodynamic assist in the event of emergency landing, this "parachute" effect would be especially useful if the hull is suffering catastrophic power failure and is attempting a "controlled" crash landing? On the other hand, I don't think MJ or any other original "official" source explained why it was there? In fact, as Whorfin pointed out above, MJ's cutaway from TMoST doesn't have it! So I would say if you want to ignore it, then do so. But I'm curious as to what you would use the space for, storage still, or cabins, or what?