I understand why stars want to make guest appearances on television shows, and I understand why television show runners like to have them on. If you're going to have your characters encounter new people as they go about their lives, wacky or serious, trite or momentous, those new people they meet might as well be a draw on their own. And there are some cases where the famous person cast for the job might also be the best person for it. If you need someone to play a super-flamboyant friend of the gay couple on Modern Family, Nathan Lane really is quite the man for the gig.
But the risk of someone being distracting in such a role, of the audience turning in for the mere oddness of seeing someone very familiar in unfamiliar circumstances, of worlds colliding. And those guest spots? Those could have been work for someone who actually needs it for financial reasons, for reasons of keeping a threadbare career extant, rather than to tweak a career that's earned its owner millions of dollars.