I never gave it much thought until someone mentioned it in a book I read the other day. The author had admitted that the heroine in her current story had been a throw away character in another book. She wasn't supposed to get her own story but the author had changed her mind.
We focus so much on the main characters in a story that the secondary characters are just used for the main characters to bounce off of, or give a little depth to the plot, and then left to fade away never to be heard from again. Sometimes they're killed off. To hear them referred to as throw away characters is kind of sad.
Unless the author is writing a series or something once the story ends so does everything else. We know what happens to the hero and heroine, but haven't there been there times when you wished there was a sequel with the secondary characters because they were interesting and you wanted to know what happened to them?
The truth is there will always be throw away characters. Interesting, compelling, humorous people that might deserve their own story but never get it.