actually, I don't see why a slipper clutch isn't the norm on all bikes. I guess rev matching is pretty fun too.
Production costs of slipper clutches, et al, aside... an abstract comparison of sorts:
Learning to ride on a non slip dry clutch, is kind of the same as learning to mountain bike on a rigid framed bike. Both require you to be smooth and fluid in your actions. You have to adapt your riding style to suit the equipment. With mountain biking, the rigid frame makes you choose smoother lines, and perfect your technical skills. When you add those skills to a suspension (front only, or dual suspension) bike, you have a much faster and more efficient "machine"...bike and rider.
Learn to ride with a non-slip clutch, and the same principles apply. Knowing when, and how, to shift and manipulate the clutch will ultimately have greater benefits when moving to a slipper unit. Again making a much faster and efficient "machine."
Just my $.02
