"Where I think this whole saga gets very frustrating for a lot of current and potential Mac Pro customers is that Apple is describing a product — a powerful, professional-grade, modular desktop computer — that already exists: it’s the tower-style “cheese grater” Mac Pro. While Apple is working away to reinvent one of the most critical components of a professional user’s workflow, those users are stuck with product choices that may not quite fit." - Nick Heer, Pixel Envy.
This should be embossed onto Apple's Professional Workflow's HQ. To paraphrase Paul Haddad, just throw some Xeons in a box. This should be easiest product release in Apple's entire lineup. Pros just want a box that can house multiple storage devices, PCIe slots, the latest I/O (even thunderbolt is entirely optional when you have PCIe) and lastly, user serviceable. That's really it. They could literally reuse the case from the Power Macintosh 9600 and we wouldn't care.
Apple envisioned the 2013 as a Mac that could be carted onto the set of a Hollywood style shoot and edit dailies on the spot with Final Cut Pro X, but conceptualizing it in an entire vacuum. While Apple takes the approach the customer doesn't know what they want, that's true in the consumer market but a massive mistake when you're dealing with professional. They know exactly what the want.
If you want evidence of the demand for such a mythical device: search 2012 12 core Mac Pro in ebay and try and name another computer. Many cost more than the current 5k iMac, new from Apple.