So, the Ringo Starrs of the TV/film world?
I just don't like to think of the Jason Herveys of the entertainment world as "worthless." Art is collaborative, and being a bit player can be just as hard as being a star. And that brother in The Wonder Years was integral to the success of the show.
But in the spirit of this thread, let me think of this and come up with some ones I would classify under your criteria.