These are truly professional workstations. The kinds of customers that actually "need" a Mac Pro, are spending way, way more on salaries than on equipment, for this to make a huge difference.
All this is assuming, of course, that the new Mac Pros live up to the hype and provide those amazing performance gains. (Hardware AND software)
Hardware is hardware, so the pretty Mac Pro box isn't worth much, when the same hardware is available elsewhere. Perhaps it makes the employees feel more important at work. That said, a couple of days ago, HN had an article with evidence that the more you make your employees feel like the company is on the edge of collapse - the better they'll perform.
Just like iOS, OS X is designed to build on very specific hardware and it is known which specific optimizations and generalizations can be made. Hence the term "hackintosh".
All this is assuming, of course, that the new Mac Pros live up to the hype and provide those amazing performance gains. (Hardware AND software)