This is best answered by a Racer / Drag Racer. I have not needed to replace them on my last bike, the '91 ST1100, since it was shaft drive. I put 100k miles on it with the original spline...
From Wikipedia:
"A cush drive is a part of a motorcycle drive-train that is designed to reduce stress from engine torque damaging other components during gear or throttle changes. A common design used by Honda, in particular on the VTR 1000F Superhawk is made of three major pieces: the wheel, the sprocket assembly, and the rubber damper. The wheel and the sprocket assembly fit together with five sections much like two hands woven together. In between the contact of the two assembly are rubber blocks. This makes it so the wheel and the sprocket have a dampening layer between them, and the rubber blocks reduce wear and fatigue of the metal assemblies.
When the rubber damper becomes old and hardens, or wears out, the changes in load on the drive chain instigated by changes in throttle position or changing gear can cause small inconsistencies with power delivery to the wheel. Loads particularly on the drive chain can be massively increased in these conditions, increasing the risk of breakage or of contact with the swinging arm resulting in damage. Perhaps more pointedly the sudden transfer of force to the rear tyre can cause momentary loss of traction (lock or spin) resulting in small changes of direction or at worst total loss of control."
Written by a Brit from the looks of it.
Here is what the ones from BoostbySmith look like:
Sorry about the picture size...
They should be inspected whenever the rear tire is off.
back to the board 
