Hi,
The plastic gears prevent damage to the motor as they DO fail if overstressed!!...for Futaba S9206's they can be sourced seperatley. I've had to replace the metal gears in my elevator servos about once a season which i thinks a good price for a) good maintainence b) peace of mind! Especialy when I'm currently putting in at least 140 flights a season on the same aircraft.
Regards
Rich