If you're going to need a machine in the class of the Pro / Precision 690 / xw6400 etc, it's a mistake to invest in the Mac Pro if your main use for it is Windows - it's a compromised machine for that purpose. I can't understand the logic of doing what's in the first post beyond a very expensive bragging right. No PCI slots for a start and the Mac Pro is more fragile as it were and more suited for domestic use, certainly when compared to the Dell Precision 690, and perhaps the HP xw series as well. It's the little things - like the separate cooling for more subcomponents, better motherboard, enhanced storage support, better support that you get with the competing products (and now at lower prices for some categories of buyer) over the Pro.
I use mine purely as a regular OS X desktop because I decided the 24" iMac was a bad idea. If a headless iMac was available I would definitely have gone for that instead of the Pro - and I suspect many people here also made the same decision. Apple by upselling simply got you to consider a class of machine you would probably have never considered if you were a Windows user - little wonder so many Macheads are happy with the performance of it, what you're buying is nearly a proper workstation.