The carbon fiber tank is used foir weight savings. Please correct me if I'm wrong, but I believe the Yosh tank weighed close to the stock tank, so there's not much in the way of weight savings.
You are correct GSXRturbo1. That might be because it's fuel capacity is 24 liters over (21 L5.5 US gal) (California 19 L (5.0 US gal)) the stock Busa tank.
It costs $3500 plus change and are only available by way of special "one off" orders from Yoshimura. Yoyodyne's Carbon Fiber tank is shy of $1100USD so the savings of weight and costs are there to be duly noted. So if like Jerry says, if the issue of our atrocious fuel eating the CF can be avoided, the CF would be the way to go.
Unless you simply wanted pure bragging rights.