UR exactly right. Plus another advantage is what the OP said about space on your cards and maybe your PC ability to handle bigger files. It is very easy to switch from DX to FX and back on the D800. Push two buttons and roll the selector. Takes 4 secs. I don't personally need the speed difference and don't plan on buying a grip until/if there is a good knock-off... as Nikons grip is $400+. The thing that convinved me on the D800 was the options to do DX or FX. For me there is more advantage to use FX and allow for better (more selective) off center crops that I might not have if I fill the DX frame.
Dan (Nikon D800,V2,Sony HX400V,Lumix ZS40) "I don't read, I just look at pictures" - Andy Warhol