I'm also one of those users who has a constant need to upgade my knowledge and skill, but I have my own reasons for buying the D800.
Don't get me wrong. I'm keeping my D700, but I've regretted selling my D90 and not being able to shoot in DX format. Similarly, I shoot landscapes, and was curious about using a medium format camera which would be way out of my league financially, and certainly my skill level could never justify my taking that plunge. Then along comes the D800 which addresses both of those formats, as well as FX. It struck me as being a bargain.
Actually, I think that qualifies as a "good" reason.
I suppose if I were to sell my D700, I could also argue that for less money than it would take to buy a D7000 I'd get 3 cameras in one. However, I'd lose the frame rate for shooting action which I get with the D700 when paired with the external grip.