Originally Posted by lucid
There doesn't need to be anything visibly wrong with the rotors. The thinking is that if you wore out your pads, the rotors experienced wear as well, and you can't just leave the rotors there until they fail. They obviously considered data on rotor wear before coming up with this schedule, so they are doing the safe thing.
Rotors are replaced when they approach minimum thickness (which is stamped on the center). Typically, they will last two sets of stock pads, but your results may vary.