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.