I road race a VC 460 with a 4 disc Bully on a jackshaft. What Brian says is true. The jackshaft does save the clutch discs. I buff the discs after two race weekends and change them after 6 race weekends. They are usually still within spec so I keep them for spares. The belt drive on the crank to the jackshaft and a chain to the sprocket configuration works very well.
I think the weight of the big clutch hanging off of the crank may contribute to abnormal disc wear in non-jackshafted applications.