I initially thought Shumpert, LeBron and JR Smith was good enough. But JJ gives them that extra lineup flexibility against GSW. Shame to lose minutes to quality big men like K-Love, T.Thompson or T.Mosgov though.
They can only afford to play one of those guys at a time versus Golden State anyway.
Adding Joe would allow them to trot out a lineup like Irving - Shumpert - Joe - Bron - Thompson/Love, which could be tough to handle.
Similarly, if Joe went to OKC, they could trot out a lineup like Westbrook - Waiters/Payne - Joe - Durant - Ibaka.
Joe would get absolutely shredded by any of the Warriors small fowards or guards and would be a complete liability for them. He could maybe help against other matchups, but warriors, no.
If he were playing on the wing, I'm willing to believe Iggy could give him a hard time, but I think Joe could handle Barnes just fine.
Anyway, especially for OKC, he'd be much better than, say, Morrow or Singler, and that's the point. Right now, OKC can't contemplate a 5 man unit with Durant at the 4 because they just don't have anybody else they can afford to give significant minutes to at the 3.