As much as Sam and Dean suffered from Jack's arrival, Cas really got screwed over the most IMO. They didn't even give Cas the "father" storyline when he was the one fighting for Jack's survival to the near demise of his relationship with Dean. He was sidelined, got beat up all the time in terrible ways, never mind the uncanny resemblance of Alex and Misha.
I had to laugh when I read a comment from Jared, that I'll have to find when I have a moment, that Jack wasn't Sam's kid. I think even Jared didn't believe in Sam being Jack's dad.
Jack might be popular with a certain segment but he sure doesn't deserve all the big kills and storylines he got after those were stripped from the 3 main characters.
I maintain that he was always the real character they wanted to do a spinoff around or at least the one Dabb wanted. I think Berens was all about Wayward Sisters and even if tht had come to pass, I think Jack would have been spun off into that show. And since WS failed, we are stuck with Jack now. What a bunch of shit!