So far as I can tell the only thing keeping Bobby and Cas alive is overwealming popularity.
And with their ways of shocking us, who knows if that holds for much longer...
I'll utter a blasphemy and say that while Castiel is a sympathetic character, and well played, I really don't see much of a point of him being part of the story. Or at least being the regular member of the cast. I really don't hate him, or want him out, I just think he isn't that necessary.
That might change, of course, if we find something crucial in season finale or something, linking him to the overall story.
My five cents, don't get angry, Castiel-fans.