IMHO there are two cases where it is:
1. You have very wide angle glass for FX that you badly want to keep using. (You can now buy very wide DX glass and get out to the same extreme limits: 8mm DX, 12mm FX. You couldn't go that wide in DX 5 years ago.)
2. You want the high ISO performance. #2
is why I have a D700. Those great big pixel wells just suck in light and turn it into images, even in the dark. ISO 800 is nothing to worry about (I'll use it for landscapes to be printed 20x30 and up), and 6400, HI.1, and HI.2 are actually usable for documentation and selected other kinds of pictures.
Going from DX back to FX cost me a mint in lens upgrades -- I had to replace the wide end of my collection AND add some longer lenses (because I'd gotten used to 200mm on DX as a regular thing).
It also has remarkably fine autofocus, though I have a hard time thinking that would be the overriding driving factor for very many people. It's fast and accurate even in appalling light (like, bonfire light outside at night).
It doesn't have video, remember. Fine with me (I do video with my EPL-2 :-)).