I think a more compelling idea for having a camera on the outside is to allow for hand tracking within the game. Imaging if Leap-like functionality were added to the Rift. Flying through space in your star ship, you could reach out and actually interact with the controls in your cockpit. I think that could be a lot more compelling than what we are seeing with games on Kinect today.
A stationary, secondary observer (like a Kinect) would be several orders of magnitude more reliable. All that's left is for someone to make a game that actually utilizes that idea.