The D3x has many advantages over the D800, body build, as someone said earlier, being foremost among them. I've dropped my D3x twice, from 3-4 feet, onto asphalt and rock. Both times it emerged unscathed (though scared the crap out of me). If the D800 is anything like the D7000, I am sure the outcome would have been different.
The D800 is at the oft'-called "semi-pro" X00 level,
as for D200, D300, & D700. Is there reasonable evidence
from usage of some manifestation of this supposed lesser
build? --even along the anecdotal lines of the alleged
weakness of the 5DmkII from that Antarctica field trip?!
Or is whatever difference might exist on the order of minor
difference such that it would be tricky to find cases where
the D<X> body would survive better than the D<XNN> one?
--e.g., dropping the camera into the drink : all hopes sink!
As for the integration or not of the grip, that can be seen
as a possibility for lighter vs. heavier --and I've seen many
folks wanting and acting on getting the D700 vs. D3 because