The plus of the in-camera DX mode (compared to later cropping): The camera meter uses only the DX area, so a bright highlight outside of the DX area won't affect the exposure.
Is this an opinion or a fact? Two days ago I decided to use a Nikkor DX 18-200 on my D700. The first time I have tried it and in a few images I had the sun in the area outside of the frame and wasn't sure if it was affecting exposure. Sometimes I "missed" seeing the frame and had to constantly zoom wider to re adjust. I was shooting fairly static subjects and static ones. I enjoyed the extra zoom possibilities but at the wider end - 18mm - I often wished I could have used the portion outside the frame. I will try again without the constrains of the frame because I could see possibilities of the extra space being cropped down to a panoramic at the wide end. Overall I had mixed feelings about the use of the mode. The ability to mount a 18-200 lens was the big advantage.