I too have just bought a D600 and will now have to sell one of my 2 x D300's (wifey's orders).
My reason was that we run a Speed and Agility training business and I need a camera that can take decent photos but also shoot HD video for when we run clinics and for general uploading of training videos to our website or youtube. It saves carrying a camera and video recorder at the same time. I needed quality video capture as well as awesome shot taking (which my D300 was pretty good at I must admit) in the one hand, When the D600 came along it was perfect timing for me.
I'm keeping one D300 because its a great backup and has a higher fps for game day sports shots, but must admit the D600 is filling that video gap for me nicely.
Full-frame was not a deal-maker/breaker for me. But most of my lens purchases over the years seem to have ended up that way anyway.