I suspect that a lot of people bought the Mac Pro despite the fact that it has Xeon processors, rather than because of it. They're a lot more expensive without a huge performance benefit for desktop or workstation applications.
The main advantages of Xeon were dual-socket support and support for oodles of RAM slots, and the new Mac Pro supports neither of these things.
I think you’re omitting the Xeon’s increased stability versus consumer grade architectures. Many Mac Pros are used to run non-stop, they need to be more dependable than other computers.
Does this really matter in 2013? Serious question. I could see ECC having been useful in 2000 in the midst of the megahertz race, but I'm not too convinced about today...
The probability of a bit error on normal RAM is quite high [1]. The more RAM you add the more likely you are to see corruption. Not that ECC fixes everything - I still see uncorrected ECC errors on the older HPC nodes on our grid.
The main advantages of Xeon were dual-socket support and support for oodles of RAM slots, and the new Mac Pro supports neither of these things.