This isn't a snide remark. It's a genuine question . Who will buy these?
I work in ArchiCAD a 3d Architectural modeling and drafting system. The base Mac Pro will probably be way more than what my software requires for at least 3 years. It is so powerful I would be surprised if it didn't last 5 years before I start wanting a faster option.
On top of that external expandability works fine for me. I can throw a 960gb ssd in a usb 3 enclosure for under $600 and get near Sata II speeds. Though I would just use what I have now for external storage.
Do we know for certain that these Graphics cards won't be upgradeable? Apple added a plethora of non-apple graphics drivers in Lion.
You are right about staying in the mac eco system though. The cost to get a comparable windows machine as well as replacing software and readjusting to windows would end up costing more.
Anecdotally I was working with an Architect who viewed PC's as a throw away commodity. Once one failed and he'd just through down a couple of hundred on a new machine. He figured on a new machine every 1 to 2 years. My 2008 Mac pro blew his brand new 2012 PC out of the water. Even though it had seemingly better specs including more ram and larger video card. To get a quality comparable PC that would last would cost so much that he, "...might as well buy a Mac" He never will though because of the hidden cost of changing over.