I did more than consider buying a backup D700 - I bought one. My first one has about 12K on the shutter, so I really didn't need a backup. However, I occasionally shoot weddings, portraits, sports, etc., so when I found a mint, used D700 with 4500 clicks, I jumped on it. For me, it was simple: there's nothing that I need a camera to do that it can't do.
I don't want video. I don't want the extra megapixels. I want 8 FPS (with a grip) for shooting sports. I want to keep using my CF cards. I want the AF-On button.
Now I don't have to learn a new set of controls or get used to the feel of a new body. I suspect I've got the camera setup that I'll be using for years.