What the Mac Pro has to be compared with are workstations from Dell and HP, not home made boxes. . . .
Apple has obviously spent more engineer CPU time than any other workstation manufacturer when designing the new Pro. I would dare to say that this is pretty much the only innovative desktop computer north of year 2000.
Dell and HP boxes aren't so much designed as specified with the components required for assembly, and badged with the manufacturer's logotypes. That's the great strength, and greatest weakness, of the WIntel machines. Because they're composed of commodity parts, at a given price-point they're all pretty much the same. But also because they're composed of commodity parts, selective upgrading is essentially trivial.
Yes, you can build a fast machine for a lot less money, but that means you need to be willing to spend your evenings trouble shooting your system. That is PC hobbying, not photography image processing.
Well, I agree with your second proposition. I'm not sure about the first. I have a strong software background, but know very little about computing hardware. Still, I had no difficulty assembling several generations of Intel-based machines. (Mostly to run UNIX, but they all worked fine with MS-Windows.) The only real difficulties I encountered involved getting the high-tolerance mechanical parts to fit together neatly. The electronics always fired up the first time power was applied, and rarely experienced any failures.
For me, at least, the real issue is the operating system. I'm partial to UNIX—and, although Apple's variant has some attributes that irritate me, I find it a lot more congenial than any rev of MS-Windows I've ever run.
Although I also confess to deriving some esthetic pleasure from using Apple hardware.