I guess most people still don't realize the big difference in terms of DR, MPs, etc.
If you are an event photographer you need to use a flash, and this determines most of your IQ and shooting style. You don't even need a FF camera for that. I've been shooting events with a Rebel and my pics put to shame those taken with a 5D2, because I was shooting ISO 800 f/4 and knew how to use my speedlite while most of people with a FF relied on their bigger sensor and fast primes to to the job without freezing the action with a flash. Result? A disaster (for them). But the point is: such celebrative pictures do not need a high DR because the point usually is to lit subject and bg evenly. Also, no one is going to print posters from such pics, so even 10MP would be more than enough. So I would say it's basically a tie, with a slight advantage for the 5D3.
If you are a studio-based or fashion pro, you will plan your set in advance and again, you will not need a high DR or more MP than the 5D3 has. You decide if you want soft or contrasty shadows, and you can take your time to frame your subject. Again I would say it's a tie, with a slight advantage for the D800 if you change your mind during post processing. D800 clear winner if you really print big.
If you are into sports or wildlife, none of these cameras is suitable due to low fps and buffer. Again though, having to choose, D800's higher MP and DR would be my choice. Its crop modes might also be useful in such situations.
If you are into landscape photography D800 is the clear winner. More pixels, more DR, more colore depth. Can't find one reason why one should prefer the 5D3 here.
If you are a PRO, both the D800 and the 5D3 would suite your needs equally well in most situations. This doesn't change the fact though that the D800 is an overall better and more versatile camera, and it's embarassing that it's considerably cheaper than the 5D3. So yes, this makes the 5D3 look very overpriced from a market perspective.
The interesting difference is if you're not a PRO. It makes a lot of difference when you can't plan your shots to have more DR and more pixels to crop. The D800 is the clear winner here, and I don't see how any amateur should prefer to buy a even more expensive 5D3.