2006 Mercedes Benz ML350 Brake replacement

The brake sensor went off on my vehicle, and I took my car in to a reliable import repair shop, and was quoted work for replacing the pads and rotors. They implied that the pads and rotors have to be replaced at the same time - that is how Mercedes makes them. Then I called my service rep. At the dealership, and he quoted me a price for replacing the pads only. When I questioned that the rotors would not be replaced, he said that they don't have to be done at the same time, and only if they need to be. What is the protocol for the brakes, and whom should I believe? Thank you for your time. M. Marker