Originally I bought my D800E with the idea that my light kit could be the Nikon 16-85 and 70-300. That would use the D800E in DX mode with the 16-85 - and either FX or DX mode using the 70-300.
The reality is I have never used my D800 in DX mode other than testing. I have looked at crops that are DX size or smaller. The D800E outperforms the D7000 and D300 in terms of image quality even if the D800 is cropped. My preference is to crop after post processing rather than in the camera. The other advantage to this approach is you can shoot a little looser and still have a good tight crop - two images in one if you will.
The only real disadvantage of using DX mode with FX lenses is that you see the entire viewfinder with a dark line creating the border of the DX image. That means the image is a little small compared to the full frame. That can be a plus since you can see the edges more easily. But generally a small distant subject remains hard to see.