>Let me restate, why is the back plate made of plastic which >can/does crack, and not metal?
The mag alloy frame of the D800 extends, as part of the unified body frame, across the back of the camera as well. However, the back has a large cutout to accommodate the large, rear LCD. Doesn't matter what end, side, front or back, top or bottom you look at on the D800 - it's mag alloy with cutouts for controls, monitors and the mount. The exterior shell sections made of polycarbonate, silicone inserts, and synthetic rubber appliques, are screwed and latched to the various receivers and studs on the mag alloy frame.
My D700 stood up to years of abuse - bumps, bangs, collisions with stanchions and railings and walls, temperature extremes, humidity extremes, accidental positioning at the bottom of a luggage heap, minor drops, and you name it. My D800 is undergoing the same abuse and taking it just as well as the D700. From my experience anyway, I think it takes some serious combination of temperature, lateral stress and/or pressure to warp the entire assembly sufficiently to crack the rear shell. A severe drop will also do it, and I've got my D700 repair bill from Nikon Canada to prove it (sigh).