Bobby and Castiel have been on the show long enough to merit being made regular cast members. I think they make the show better. They may not be in every episode, but I think they would add depth to the stories.
I like DJ Qualls (Garth), but I think the original Bobby must be considered first.
Jim Beaver was offered the option of becoming a guest star, but he turned it down b/c it would have meant uprooting his daughter and he didn't want to do that.
Moving this to the characters forum.