Rotors need replacement if they are scored from the brake pads.
Ideally, a rotor should be a perfectly flat surface. The brake shops will use a lathe to true the surface if the rotor has just normal wear. Only so much thickness of the rotor can be removed. If the rotors have been scored from the brake pads wearing down to the pad's metal backing (metal-metal contact), then it is most likely too deep to resurface.