A consumer-grade i7 you buy off the shelf is
not the same as the Mac Pro Nehalem processor.
As for performance: yes, you can build something equally fast for less money. That was never my point.
You can a Honda Civic as fast as a Mercedes AMG. But at the end of the day, you're still driving a Civic, andthe Merc is still a better vehicle. Speed is not everything.
Robust != fast. The processors in the Pro are server grade, and designed to deal with constant, high-load work. A consumer-grade processor is not. I'll bet even money the Pro lasts longer than the consumer products without failing.
Again, yes, you can build or buy something faster for less money. But you cannot buy a
better system for much less. Take a look at similar offerings from other manufacturers like Dell or HP. They're in the same price range as the Pro - they are not $1000.