i don't think Starring Role has ever been fully qualified. My understanding is that its primarily intended purpose is for use in anthologies, where each episode has its own unique "Stars." However, very few editors use it that way. Myself including.
I've seen it used for people who don't get a show star credit, but still appear pretty regularly. I've used it for the secondary "stars" of Star Trek (first season DeKelley, and Doohan, Takei, Nichols, Koenig, Barrett, Whitney). Ben21 uses it for Rhys's character on Torchwood.
I believe right now, for shows without an editor, if you want to add a main star to a single episode, submit them as a Starring Role and they'll automatically go into Show Cast for that episode.