But you can frame at 450mm on the DX vs. 300mm on FX. Having the items appear both closer and as they will appear(or at least closer to what they will appear like) in your final photo can both be advantages. Plus you would already have to crop 33% to get the equivalent DX FOV. What if that is not enough? That is, what if you wanted to crop the DX frame, ever crop you are losing quality and potential print size.
>Or you can shoot in DX mode.
at 10.5 MPs. Plus not many are fans of shooting FX in DX mode and trying to stay in the lines. In fact, I don't know any FX shooter who ever uses their DX crop mode.
I wouldn't say that either of these solutions are better than just shooting a DX camera if the extra reach is very important to your shooting and you can't afford a lens based solution.