Yes, it's more expensive than a 5D Mark II...but it has 95% of the flagship auto-focusing system, which is not cheap. It also has a much-improved sensor (which you'd expect). And it has a host of other new features. Even though I don't like it, I can see and understand the extra $1,200 (+/-) cost.
So in my mind, the cost for the camera is justifiable. However, I suspect the upgrade cost mat not be justifiable for some 5D Mark II shooters.
Let's look at a photographer that primarily shoots landscapes. They care more about IQ at low ISOs. They don't have to care about the autofocus system. And they certainly don't care about high ISO performance. For the landscape photographer: probably not worth the extra $1,500-$1,600 to upgrade.
If I were a wedding photographer, it would absolutely be worth it for me. An extra two stops of low-light performance means better shots for my clients. A flagship autofocus definitely means better images for my clients. And the 6 frames per second will really help certain events I shoot throughout the evening. Check. If I were a working wedding photographer and funds were there, I'd absolutely upgrade.
Sports shooters? Well, while the 5D Mark III certainly has some great features, full-frame functionality is not that important and I can buy two Canon 7Ds and a $1K piece of glass for the same price as one Canon 5D Mark III body. And if I were a true sports pro, I'm already comfortable investing tens of thousands of dollars in glass...so I'd probably have my sights set on the Canon 1D X.
Studio photographer? Well, I'm probably going to stick with my 5D Mark II because my needs are somewhat similar to a landscape photographer. Frames per second doesn't matter. Autofocus doesn't have to be top of the line. And I wouldn't care so much about high IDO performance.
If money management is important to me, then the situations above generally apply. If I shoot a combination or all of the areas above, then I have to make a weighted decision. And if money is flowing and I have to have the best and latest thing...I would probably upgrade no matter what.
These are my current views...