Why trash the rotators?

Chief and AAR, you must think I'm smarter than I am to get and answer from those responses. Dan that makes sense, so would the rotators not do the same for carbon biuld-up? The lead used to cushion the valves, but it built up and caused problems? So did it build up on other areas too, ring lands? I do know that we have hardened valve seats now, I'm just tryng to get a handle on why they would be bad, my thinking is like rotating tires, you would get more milege from a part rotating verses one wearing the same spot constantly? Maybe some people would like to discuss something different and I could learn a little something.