I actually really agree with Jerry, but I still think it's all about budget.
If Donald Trump were posting on here saying "what camera should I get?" each and every one of us would likely tell him to buy a D3 or D3X and all three lenses of the holy trinity, plus a really good macro and a really nice fisheye. Why? Because we know this would be a drop in the bucket for him, so why the hell not?
However, both Donald Trump and our OP here would probably have the same issue... while they most certainly would be able to work both that camera and a lesser body effectively, there would be more to learn and to really make the camera sit up and bark on command.
If you consider how long it takes to learn photography (even when obsessing over it) that
does make the more expensive camera seem a little crazy, but then if you really have the money should you buy a lesser body?
I think the only time I would say yes here is if the person was more concerned about making a transition from a camera with more "auto" capabilities... but at that point, even the D100,D200,D300 are going to throw you for a loop, so the argument between a D3 and any of the lesser "full body" cameras is really kind of pointless.
I will say that I have made transitions between D100->D200->D300->D3, and yes... there was a bit of learning the eccentricities of each model, but I was able to use each progressive better model well from the moment I picked it up, so while there are more capabilities in the better bodies, my lack of immediate knowledge of them did not limit my abilities so much as give me more headroom for things I could grow into as my knowledge expanded.
I'm a little tired so I realize I'm winding, sorry... consider all the above as data points.
So factor in all those datapoints and I come to this...
It's all about budget.
Assuming someone is ok with not having auto modes, they may as well buy the best camera in the line that they can afford. It will give them more capability that they can grow into over time, and by and large will be just as difficult to work the basics as any other full body camera in the line, with the notable exception of being a bit heavier. Certainly, the higher level body camera in the hands of a seasoned professional is going to produce WAY better results, but it's not like the unseasoned guy is going to do any worse with the D3 than he or she would with a D300.
It's all about budget.