my rule of thumb- if you're not sure whether ECC memory is critical or not, you don't need it.
Some people do, but not everyone.
Good rule.
For me, I did buy an E5-1650v2 system (T3610) both because it could support 128 to 256 GiB of RAM and because the RAM was ECC.
I don't need ECC because I run critical tasks that would be compromised by a single bit error, but because I don't want to wonder if some corruption, app or system crash is due to a memory problem.
With ECC - a memory error gives me a BSOD with "memory error" clearly stated as the cause.
Something else to consider is the motherboard in this thing. It was a pretty radical change from a design standpoint. Usually they like to recoup costs associated with development before they have to re-tool things. --Someone else mentioned mac mini being long in the tooth...could be the reason.
Another thing to consider is while the new socket may be faster, a new 6c version will still lose to a 12c old version in practical computing.
A couple of comments....
First, motherboards are dirt cheap to design - it's all automated. In no way is Apple constrained by "it would be too expensive to redesign the motherboard". Look at the variety of motherboards that Asus, MSI, ... produce - if it were capital-intensive they couldn't afford the breadth of their offerings.
Second, the same motherboard could work with the Haswell-EP. (The Haswell-E has been shipping for a long time, the -EP is the one for the new MP6,1.)
Why? Because the motherboard is the little round thing at the bottom with the C602 (AKA X79) PCH. The processor is on
a daughtercard - which obviously would need to use the new socket. Searches on the web offer conflicting opinions on whether the C602 will work with the Haswell-EP.
Third, much of the engineering effort on the MP6,1 was dealing with packaging and thermal issues. You don't have to start over from square one if the new CPU uses 10 watts more or less - it's a minor parameter adjustment for the CAD model.
Just to let you know there isn't any competition to Intel out there... and I don't see yet the day apple will release anything with more than 2 sockets... so exit the Xeon E7...
It looks unlikely that Apple will ever release any new system with two sockets.