"identically specced" Only for very liberal interpretations for "identically specced". The problem is that when you actually try to build one identically-specced, in some cases, you'd find you spend more money on a PC than a Mac. There are specs you may not care about: small form factor, workstation processors, etc which may drive the price down. However ignoring them means you don't have an identically specced machine.
Take for example the cost of the video chips in the Mac Pro. It is actually cheaper to buy a Mac Pro upgrades than discrete cards. The D300 cards are roughly equivalent to the FirePro W7000 (~$750) while the D500 is almost equivalent to the W8000(~$1250). The D700 is roughly equivalent to a W9000 (~$3200). The prices are newegg prices. To upgrade from D300 to D500 is $400 on Apple. If you had two W7000 discrete cards, the upgrade price to dual W8000 would be $800. To upgrade to D700s would be $1000. To upgrade from dual W7000 to dual W9000 is $5500.
Now you make say you don't need workstation level cards, but that's the problem with your argument. Using a consumer level card would be cheaper; however, a Mac Pro is not designed for consumers. It's designed for professionals.