Nikon, in its long and rich history, has always developed lenses for its 35mm cameras with the F mount. When Nikon started making digital SLR cameras, they all had the DX sensor size format, but Nikon didn't really make any lenses specifically for it for quite a long time—photographers with existing F-mount lens collections used those lenses. I think some of the first DX-specific lenses—apart from the cheap kit lenses—were the 12-24mm f/4 and 17-55mm f/2.8. Until then, no rectilinear lens gave an actual ultra-wide angle of view on DX, so the 12-24 filled that void.
Since then, a lot of quality lenses have been released for DX, both from Nikon and third-party manufacturers. While Nikon still probably tries to push the high-end towards the FX format, DX still offers quite a usable selection for demanding photographers. Take, for example, a kit that consists of the Tokina 11-20mm f/2.8 and Sigma 18-35mm f/1.8 or the new Nikon 16-80mm f/2.8-4, add an FX telephoto (70-200mm f/2.8 or f/4, or 80-400mm AF-S) or a Sigma 50-150mm f/2.8, and you've got a kit that covers all the bases really, really well. Of course, that will be more than many will actually need—not everyone needs complete coverage from ultra-wide to super-telephoto.
However, Fujifilm has a far easier job of creating quality lenses for APS-C, as it is Fuji’s only format. They’re not trying to sell anyone on any other format, so the XF lenses are unashamedly high-end glass, with many in the lineup priced around a grand. I’d say the Fuji lens system is really the best lens system for anyone who doesn't need a super-telephoto (though one should come next year). That’s mostly on-paper, though—whether or not the lenses are fast enough at autofocus, or that the body they’re mounted on gives the necessary autofocus capabilities for whatever it is you shoot, is another topic. And for most photographers it probably is plenty good enough, but the rest would currently be better served with a camera like the D7100.
The question that has to be asked is,
are your current lenses letting you down in image quality in any way? Do you see a lack of sharpness, that isn’t due to missed focus, motion blur or camera shake, a defected lens, or a lens that needs AF adjustment? Do you see any problem with color due to necessary color fringing / chromatic aberration removal? Do you have any noise issue in the corners of the image, due to correction of unwanted vignetting?
If you do, get a new lens.
If you don’t, stop thinking about gear and try to have more fun shooting.
It is actually an advantage using an FX lens on a DX body in many ways, because the best part of the glass is in the center, which is what the DX uses when your mounting an FX lens.
While it’s true that you’re effectively “masking out” the weaker parts of the lens, you’re also magnifying its flaws, and demand more resolution out of that portion of the lens (if the pixel density is higher on the DX sensor, which it usually is).
So as far as lens selection, sorry but the Fujifilm X-E2 can't even begin to hold a candle to the D7100.
That depends. If you’re looking at super-telephoto lenses, then absolutely, Nikon reigns supreme there—Fuji doesn’t even have one lens to compete on that front. Nikon also has better options for macro, though Fuji has announced development of a 120mm f/2.8 lens capable of 1:1 magnification, that sounds promising. But if you look anywhere else in the lens range, I’d say Fuji is at least as good, and often better.
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