I am confused by the whole rotor wearing thing. I have replaced rotors on a car but that car had about 150,000 miles on it and I ran it pretty hard, but I have never replaced rotors on a motorcycle due to wear. I've upgraded to stainless steel on my racebike but that was for performance reasons. And I replaced a couple of rotors due to crashes but I would think the rear rotor should pretty much last forever unless you are very large and/or drag the rear break and/or have a very large passenger riding full time. That's my opinion.
Because of this thread I went and checked the rear rotor on my bike and it is noticeably grooved and I never use the rear on the pavement but I do on gravel and off-road. I've noticed the rear brake on my bike also has a tendency to lockup . Could this be due to a worn rear rotor? Could this be another one of BMW's ways to reduce costs with substandard components? It's not that expensive to replace with an aftermarket fix so I feel pretty lucky with only having to spend around ~$180 to take care of it.