How does this make sense? Why would I want a $2000 plastic 5DII equivalent when I can have the real thing for the same money? And I don't believe there is any noticeable difference between the 5DII and 5DIII sensor. So what gives?
If you don't think there is a noticable difference between the 5DII and 5DIII sensor then maybe you could tell me if you notice a difference between these photos:
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That's a back to back comparison with the 5D3 resized to 5D2 size. The 5D3 sensor has a significantly better AA filter that is more efficient in terms of preventing lost detail for a given level of moire reduction and so produces sharper images.
Furthermore many of the people saying that the 5D3 isn't that much better in ISO are not comparing the cameras correctly in likely two ways. Camera manufactuers generally make up their cameras ISO ratings out of thin air. There are a few acceptable ways of rating ISO but most manufacturers chose fringe methods which aren't widely respected so the can make up their ISO numbers as they see fit and when the 5D2 was released the ISO ratings were very optimistic to say the least with ISO 12800 being actually around what is commonly accepted as ISO 7000. With the 5D3 Canon has been less optimistic so ISO 12800 is actually a "true" ISO of 10000. The point is that people are comparing cameras using their RATED ISO which are on different scales not ISO measured on the same scale. It's like comparing a car going 0-60 kph vs a car that's going 0-60 mph, which doesn't make sense. The other issue is that after speaking to several Canon Techs and Reps, it seems that Canon specially designed the 5D3 to produce very low noise JPG's for photographers that have to shoot in JPG. Part of the innovation that they introduced was software but they also engineered the hardware to help in the process to make the noise coming from the sensor have far less speckle noise and have a very gausian distribution. This makes computer programs able to distinguish from the noise easier so when you apply noise reduction there is a slight advantage to the 5D3 of around a quarter of a stop more than the RAW data would suggest.
In any case here's a comparison between the 5D3 and the 5D2 rendered at the same resolution with the same ACR settings applied at a true ISO of 10084 for both cameras, this is a combination of a series of exposures using a method developed in consultation with the Cambridge Signal Processing Lab. I do consulting for a few camera review websites to develop testing methods and put this together for a project, with permission from all collaborators.
In any case feel free to spot the difference between the 5D2 and 5D3. 
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Look, that's all fine and maybe I should have said "noticeable difference for all practical purposes" or something to that extent. Clearly the 5DIII is a positive development on many levels. I wouldn't mind having one. And I can see that there are applications and user types who need exactly those improvements.
All I'm saying is that to me personally I see no reason to sell my MarkII and shell out a significant amount of money to upgrade. Hi ISO stuff, AF, and all the other improvements are not important enough for me and I have yet to see a real life photo where any one would say, wow, this was shot with camera A, B or C. They are all very very good, no doubt. None is "perfect".
So everyone needs to balance features, little quirks and cost of course. I personally would want a smaller, more plasticky camera for the minuscule improvement in the sensor functions that at least for my real-life applications wouldn't make any difference. Honestly, the 5DII is small and plasticky enough as it is. I actually added the battery grip to add some weight for balancing things according to what I'm used to and prefer.
And as I said, the biggest selling point for a 5DIII or 1DX would be its compatibility with the new flash system. But that adds even a LOT more money.
Considerations for people with other priorities or people new to a system may look entirely different and I'm not saying that's not valid. I'm just struggling with the above rumored features for that kind of money. 2k is hardly "entry level".
