I'll reply with an answer that applies to my specific case. Might apply to other DX shooters, might not.
I'm an amateur hobbyist, and I'm not going to own a whole fleet of cameras. I just need one, although I'll keep the old ones for backup, and less demanding uses. Right now my main camera is a D300, which I like because of the durable body, weather sealing, and control layout. I might not need a "pro" body, but I like those features of the D300.
A D600 would not be a good substitute for me because most of my lenses are DX lenses. I prefer them lots of times because they are smaller and lighter, and it's also nice that they are less expensive. Some of my lenses are higher-end, and I could use them with a D600 when I need something like that, but I don't want to give up my DX lenses, and I want one camera that will work with all the lenses I'm using. Also, for sports I like the "reach" of DX. I know I could get this by cropping a higher-mp FX image, but then I'd need to replace the DX lenses I want to use.
If I were changing formats, I'd be more inclined to go to M4/3 than to FX. I'd have to buy new lenses either way. But DX is a good compromise between image quality (I love what I get from my D300!) and portability. I just like the higher-end camera.
Most Nikonians are probably more dedicated photographers than I am, but I'm guessing I'm more representative of the broader market of people who actually buy cameras.