Yoav

The metal-gears themselves are more rigid and can survive sudde shocks better (like when hitting the control-surface going out the door or something). But, when the control-surface is shaken by engine vibes, the plastic gears that are a bit softer flexes slightly and dampens the vibes. When metal-gears are slammed together there are no damping, just metal against metal that is not helping to reduce the vibrations. Thus the pot sees more high-frequency vibration that wears it out faster.
Also an issue is that modern high-speed, high gain, high accuracy digital servos often can start oscillations when the pot's gets worn, it seems to me that this is a bigger issue with metal-gears, again bechause there is no damping of the vibes. Oscillations wears the gears and the pots even more and also severly increases the power consumed by the servo.