This is something to consider for DX users who are focal length limited i.e. you're shooting with your longest lens and still cropping from DX.
FX has a number of advantages, but if you crop deeper than DX you negate all of these, and are left comparing tech-for-tech where the D300 will still suffer somewhat due to being 5-year-old tech.
Anyway if that's your goal then the D7000 is a much better target, since it can give you that newer tech with a higher pixel density. But, I can say owning both that the D7000 is in no way a replacement for these very kinds of subjects (e.g. fast-moving, distant birds) so I don't expect D600 to be a better camera in crop mode either, even if it might push the D300 in image quality. You won't get the control layout, buffer basically forever, 51-point AF, etc. that keep D300/s users waiting for that D400.