However, surely different photographers will put different relative weights on the image size disadvantage vs the "peripheral vision" advantage, so I do not see how you can categorically declare that one "offsets" the other as if the VF image size deficit is rendered irrelevant.
Of course, BJL. We're not talking about objective, non-subjective facts here, such as the resolution of a lens in terms of line pairs per mm at a particular MTF. All models and all formats of cameras have numerous advantages and disadvantages when compared with each other, and such advantages and disadvantages will tend to have a strong subjective element which is affected in general by the circumstances, experience, desires, goals and shooting style of the photographer, and in particular by the requirements for a particular shot at a particular moment of time.
We both agree that the smaller size of the view of the intended composition, as seen through a 135-format viewfinder in DX mode, is a disadvantage. And we both agree that the consequent effects of being able to see in that viewfinder parts of the scene surrounding the intended composition, is an advantage.
Unfortunately, there are no objective units of measurement which allow us to quantify such advantages and disadvantages and put them on a scale to see which is more significant.
Regarding the D800E, the main advantage of DX mode, in my subjective opinion, is when using a DX lens. The black-lined, DX-size frame in the viewfinder should be a useful guide to indicate that image quality outside the frame will likely be very poor.
Since I don't own any Nikkor DX lenses, I've never used my D800E in DX mode. I prefer to crop to taste in post processing. For me, the main advantage of using a D7000, for example, in addition to a D800E, would not be because of the enlarged image seen in the D7000 viewfinder, with a give focal length of lens, but because carrying two such cameras, with zoom lenses attached, can save time and hassle in changing lenses.
However, neither of these two advantages, the greater magnification and the avoidance of changing lenses, is sufficient reason for me to carry my D7000 as well as my D800E when I go out shooting. I need a greater incentive, and I'll have it with the D7100 which provides a 50% increase in pixel count compared with the D7000 or the D800 in DX mode.
Now, it's understood that a 50% increase in pixel count is not a 50% increase in resolution. It's only a 22% increase in resolution, but a 22% increase in sensor resolution is probably, on average, about equal to the increased resolution that a good prime lens has compared with a moderate quality zoom.