[djpateman]

Originally Posted by RWebb

I thought the pads would just wear down to match a shallow, rounded groove pattern? i.e. only sharp or deep grooves req'd new rotors...

They will, given enough time. In the meantime do you still want to be able to stop effectively? More than that, the rotors are often warped, glazed or work hardened, and the machining gets rid of those problems as well.

Rule of thumb: if it is time to replace the pads, then it is time to resurface or replace the rotors.