Well, theoretically, protein skimers also remove trace elements. So like use of something like a polyfilter or other resins, I probably wouldn't dose and then add a fresh thing of carbon or a polyfilter or something...which might be somewhat counterproductive. But the question of the carbon and trace element thing is a pretty active debate.
Its generally not run all the time because in theory if everything is running well, it should be needed all the time.