Take another look at your frame. The shaft drive is going to want to be placed right where your chainstay is currently located. Notice on the pictures posted above the chainstays have been moved higher on the seattube and dropout. Shaft drive requires a frame designed for the purpose.
An enclosed chain case has many of the same advantages at a lower weight and probably lower cost. However a shaft drive is probably still lower maintenance and less susceptible to environmental conditions. I'm guessing these bikes are nearly comparable to derailer equipped in performance. I doubt there is much of an efficiency loss due to the shaft drive.
Craig