As I have tried to explain before on a different thread, the advantages of the high resolution have very little to do with ultimate print size. (I am not saying it is of no use to someone wanting jumbo-sized prints, although this can be over-stated as the larger the print, the greater the distance at which it should be viewed).
No - the immense advantage of the D800/800E is the 45Mb lossless compressed Raw file. Only a luddite would deny that modern photo-processing is data processing, pure and simple. So the more data you have available, the greater your photo-processing options.
Personally I never print larger than A3+ (I only use an Epson R3000 printer) but I might print that size from less than 10% of the image file. Another huge advantage of the D800/D800E is that you can create much wider panoramas without the tedium of stitching, simply by shooting with an uw/a lens and top- and tailing the image.
It is only when you get into that level of processing that the enhanced detail of the D800E over the D800 becomes obvious - but when it does, it is very impressive.
For me (and others will have different needs/views), the big bonus I get from those cameras is a much higher level of flexibility. It is now not unusual for me to create half a dozen quite different images from a single exposure. Before switching to those cameras I would have said that photography was 80% science and 20% art. Now I would suggest 60% science and 40% art (because they have made me less scientific and more artistic in my approach.
But, of course, everyone is different and it is very much horses for courses, strokes for folks, or whatever other inane cliché you might favour.