What I've heard is that JS is the determining skill on all shots. As you get farther away from the basket, a penalty is placed on your JS effectiveness, which of course makes sense--a 5 foot jump shot is easier to make than a regular three-pointer or a half-court desperation heave. JR mitigates that penalty. So to answer your question, let's take a guy with 4 JS and 20 JR shooting three-pointers. The 20 JR would mean that the penalty for distance is inconsequential, so the only real effect would be the 4 JS, which still wouldn't be enough to make a whole lot of shots. So basically, JR will ultimately only allow your player to be as good as his JS, and if that's low, JR doesn't really help much. That's why most managers prefer higher JS than JR. I'm not sure if I'm expressing myself clearly, but does that sort of make sense?
I'd find the thread I saw this explanation in, but it was in an older thread I found when I had too much time on my hands and I don't have supporter so I can't search the forums to find it again.