Bare in mind that the 5DIII resolves nearly as much detail as the D800. It's only the top end optical resolution of a few of the worlds sharpest lenses which can allow the D800 to out resolve the 5DIII and even then, there isn't much between them.
1.) I have both cameras, and the D800E clearly
produces more of the fine detail that renders an image more realistic (if all of the resolution-saving techniques are used and the image isn't bludgeoned to death in PPing).
2.) This resolution differential can clearly
be seen when using any lens in my bag and most Canon, Zeiss, Sigma et al lenses in current production.
3.) This resolution differential can clearly
be seen in downsized images (as small 1800x1200 px for instance).
4.) Many folks "clearly
" do not care about or even see the fine details in the natural world.
Now, whether or not "clearly" equals "nearly" is a probably matter of observational skills.