Indeed, not all of my lenses can be used on FX. But I am not planning on switching completely to FX. D7100 is perfect for birding and macro, which I like. I am evaluating an option of supplementing D7100 with D700 (keeping D700).
One interesting benefit of having FX and DX is that each of my FX lenses now has two effective zoom ranges (50mm is now 50 and 75, 35-70 is also 1.5x35-70). How good of a reason is this for having D700?