Well, I'll use an FX lens on a DX body and enjoy the benefit of increased EFFECTIVE zoom, if I'm using a long lens, but if I want to use a wide angle lens that causes me to lose the wide angle part that is what makes a wide angle lens so fun to begin with! And doing the reverse, DX lens on an FX body, I have to use crop mode and lose the use of about half of my 36 megapixels.
I see these mix-and-match combinations to be mostly a matter of accepting some severe compromises. But I will say that getting extra zoom out of an FX lens on a DX body can at times be a GOOD thing. I would call that a benefit. My Tamzooka on my 24 MP D3200 vs. the same lens on my D800 works out to be pretty much the same thing. But that huge lens on that tiny D3200 body looks kind of silly. Seems to work fine, though.
I understand that it was probably smart of Nikon to introduce the DSLR with the capacity to use MOST of the existing Nikkor F mount lens catalog. But then again, the issues involved with FX/DX pairings can be a little bit of a headache as well.