Yeh the carbon shaft is mighty expensive. It can be very light, but mainly I like it's unique properties, like the fibres actually twist and tighten as torque is applied, giving a little springs to the drivetrain, like I guess a belt final drive on a Harley does as opposed to chains. Also, they are said to have inherent safety in that once they break they unwind and brush themselves to death, they polish the underside of your car, unlike metal which turns into a flailing stick of doom. I have heard however that if scored they will weaken substantially, and there's a more chance of running over something (a speedbump even) on the road than a race track.
Anyway, I thought you'd laugh at the idea, but am glad you gave it some thought.
70vert, two benefits of central discs I can think of are mass centralisation and reduction in unsprung. It would look wierd though with open wheels - nothing there behind them. Putting new rotors on looks like it would be a pita to do.