What, exactly, was the point of the guest stars on “Changing of the Guardian”? Obviously, The Simpsons has the power to just bring celebrities on for no good reason, and constantly, but this week seemed uncharacteristically egregious, especially given how much Danny Devito’s comeback as Herb was hyped in the marketing. And then he just shows up for one line? Maybe that would have been fun as a little surprise, a nod to the show’s history, but it still rubbed me a little wrong. And then there’s Rashida Jones, who is given basically nothing to do for the entire episode.