JR is a novel addition to the team because he's the closest thing to a pure shooting SG that we'd have, and he has legit range.
These qualities will be made more important when Melo and Amare are put back into the lineup.
Nobody should be sold on Fields as our starting SG, either, IMO -- especially when Melo/Amare are back.
Shumpert is still raw and prone to inconsistency; JR will allow us to use Shumpert more sparingly, perhaps, but also more effectively.
We need as much talent as possible; we could use a real shooter from the 2, with range.
If JR acts like a punk and plays like shit, he just won't be played. And he's going to be getting paid 2.5M on a team with Tyson Chandler, Amare, Melo, Lin, etc. If he acts like a punkbitch, the only ramifications will be him being branded a punkbitch.