I think that Fields should be starting not because he's superior to JR or Shump, but because they are better suited for the bench at the moment.
Shump is a high-energy guy, still prone to mistakes and better off when he's not asked to do too much. JR Smith may be as good a defender as Fields, and certainly has a much better outside shot. But JR's instincts are plainly to shoot first, ask questions later, whereas Fields is continually moving the ball and only shooting when he's got a clear path. And we need JR's offense more with the second unit.
None of this is meant to say that Fields should get more minutes in the entire game. In fact, I think that Shump's defense will often warrant that he be the guy finishing the game and playing the crunch minutes.