True. However the D600 has almost no amp noise even for exposures several minutes long. At least for reasonable sensitivities and when the difference between camera sensor and ambient temperature is small. Moreover, when taking astrophoto pictures it is always better to take separate light and dark frames, average them and remove them in the post.
The huge disasvantage of in-camera Long exp NR is that the taking of pictures takes too long. Also enabling the feature halves the depth of the image buffer even for short exposures. Thats why always disable it.