There are a lot of misconceptions in this statement, most of which are clearly explained in this
blog post from Nasim. You do not "get closer", you just get pre-cropping. Yes, with a D600 you lose mexapixels, but you gain IQ. I spent a week in FL shooting birds with my D600 and my D7000, swapping my Sigma 150-500mm back and forth. I was far happier with the D600 images I got than the D7000. I shot in FX mode, and in many cases needed to crop to DX sizes or smaller, but the images were amazingly sharp and given that I'm firing at ISO 800-1250 with birds in flight, the difference in noise was beyond significant. I had little to no noise worries, whereas the D7000 at ISO 1000 immediately gives me noise to deal with. 90% of what I shoot will never be shown anywhere but a computer screen, and I can certainly get an 8x10 out of a 10MP image.
If I ever sell one of these to someone who wants a huge print, the money made will help pay for some good upsampling software.