I've thought that before, but theres one reason why I'm GLAD they didn't:
If you remember, Tara was never billed as a regular, even though she had enough screen time and enough role in the plots to be qualified as one... HOWEVER, she was billed as a regular in that ONE episode, so I can consider her a full on Scooby because she was in the credits. Based on that, I would also have to consider Johnathan a full on cast member if he was billed as a regular in one episode. Now this is probably just me, and probably none of that made sense, but thats my thinking.