Fiber gears are fairly common to many applications: I'm a projectionist and use machines ranging from the early 1940's up through the late 90's, and every variety of 35mm machine I've used barring one uses fiber gears meshing with steel. Seeing continuous use, these gears last the life of the machine in most cases. The theroy of use in this application is that should something other than the gears seize the drivetrain, for instance a "Wrap" or jammed film, the fiber gears strip while the motor continues to run the projector drivetrain, preventing considerable damage to precision componets.
I was not surprised to find a fiber gear in the Volvo, and take it to have been designed as a precautionary measure with the intention of saving the engine of major damage. Consider what could occur to the parts moving at high speeed in the engine should something introduce enough force to arrest that mothion... Having a readilly accessible stress point seems really very reasonable.
For an engine which sees regular use in the high performace range, the fiber gear may be under-engineered, and the steel preferable in that case.
As for myself, should I ever manage to get the car to attain ANY speed, I'll keep a fiber gear on hand, just as we do for the projectors.
-Sean