A lower base ISO can be useful esp for controlling shutter speed, so ISO 64 is a welcomed move. Also you can help out flash sync speed with a lower ISO (though HSS does fine in many cases).
Offering a base ISO at 64ISO has another value for Nikon.
Although the D800 has amazingly good high ISO image quality, it is at base ISO that it is offering an image quality far superior to that of its competitors.
Yet, it took actual shooting or measurments to see this value. It was not visible in any specification.
By re-designing a sensor with a higher photon well capacity, and therefore a lower true base ISO, Nikon managed to show in its spec sheet the superior image quality of the D810 at low ISO.
We will have to wait for DxOMark results to see how it translates into real world performance, but the mind share has already been captured. This may be a good example of marketing driven engineering.
Cheers,
Bernard