I recently covered rubbing, cogging and shorted PMAs in a previous post.
Above is a picture of a rub caused by corrosion swelling in the steel laminations behind the ferrite magnets. Such corrosion will tend to push out the magnets and cause a rub. Contact with the stator then results in the wear (polishing) of the magnets.
Check for signs of this after each service from 5 years of age. The corrosion process can be slowed by soaking the rotor in fish oil overnight. Replacements rotors start from $49 US.
The rotor in the picture is about 15 years old, so it has had a good run.
Low power turbines (or those run intermittently) are more likely to suffer corrosion as there is less PMA heat to drive out moisture ingress.