You need to compare apple-to-apple, like no one says a compact is a crop version of a FF or DX camera, although if you follow the arguement it is indeed so. That's why with a focal length of say 6mm you are getting FX FOV of say 24mm or 28mm.
My point is, to compare apple-to-apple, and to say DX is a crop version of FX, you need to hold something constant - in this case the pixel density. A more-or-less valid comparison would be between D3x and D300s, cause with D3x and DX cropping you get roughly the D300s resolution, with high ISO and noise performance being similar. Then you are really just cropping out the FX into DX.
If you compare D3/D3s/D700 against D300s/D300/D90, technically you are not comparing apple-to-apple, cause a DX crop from the FF cameras will get you only ~6MP, while the DX cameras give you 12MP. It is not a crop from FX, so long as your lens resolution is not exceeded. You have a higher pixel density in the DX cameras, so if you want more "reach" and can deal with the ISO and noise performance, it is indeed an advantage to have DX.
So go ahead and shoot a safari with a 300mm on FX and tell the guy with DX and the same lens his shot is a crop from FX. The DX guy is likely to have sharper picture with more resolution if the final image is the same size from the same lens. For the FX guy, he will need 450mm lens to outdo the DX guy with 300mm. In which case the FX's better high ISO and noise performances will produce better shots than the DX, and only if the ISO goes above certain critical values.
So with 12MP FX, you get better high ISO and noise performance, but you also need longer lens to get the same reach of DX, and there is the limitation of good FX lenses vs good DX lenses - a lot more good DX lenses cause DX uses sweet spot of all lenses, mostly lighter and cheaper.