Higher Friction CoEfficient...
Since the SC was meant as a road car, the engineers needed a trans that could handle the torque capacity of the engine (the mustang one supposedly didn't make the cut) so the engineers turned to the Truck transmission. But in order to acheive quick-flick shifts that one would expect from a road car, rather than the long drawn out and deliberate throws of a truck, they switched to the fiber blocker rings.
It works sort of like an fiber brake pad against a metal brake disk each time you move the shifter. If the brass blockers were standard on the SC, they would have lasted longer, but how well does a metal brake pad work against a metal brake disk?
Drivers who have converted the brass blocker rings over the SC have reported Gear crunching in 1st and 2nd gears when speed shifting. A fresh set of Fiber blocker rings allows the driver to complete this task with no noise. Problem is they wear out in about 30,000-50,000 miles.