Of all the things to improve in photography the camera is the least important as you say. Upgrading self, technique, and lenses end up being bigger wins in that order. Even so, I wouldn't think less of people who upgrade because it's fun for them. So long (in my opinion) as they are using discretionary funds that aren't taking away from their family or personal responsibilities and obligations.
But different people have different motivations. I certainly don't really need more than a D700 or D300 but I might upgrade anyway. Part of it is that I am a bleeding edge kind of guy (it's a common personality type of us software developers) and part of it is that I don't have a family so I can afford to play if I want to, lol.