When you have drilled/slotted there is a break in the metal so it goes from one spot to a high spot with out the incline and the cutter hits it straight on and breaks the cutter. If it tries to cut too much it just slows down or stops. When you have a high spot on a regular rotor the cutter will cut from the low to high at a gradual rate.
I would like to hear why someone would NEVER turn a rotor though.^^^ The lathe doesnt care if it has holes, it will just make it flat. Throw away society we live in but the rotors can be turned just like any without being drilled and slotted.
