I enjoy a vast library of games, upgrade my computer at will and have the ability to choose from a vast plethora of third party accessories. Now, these things are all possible with a Mac, this is true... but 'functionally identical' is a stretch.
functionally identical isn't really a stretch any more.
One of my computers is a 3.4 GHz Intel Core i7 with 12GB of DDR3 RAM and runs an AMD Radeon HD 6970M
The other is a 3.2 GHz Intel Core i7 with 16GB of DDR RAM and runs geForce GTX 460
Which one is the Mac? Why is it not functionally identical?
Unless you mean that the PC can't run OSX, which is true.
Oh and in before upgrading components:
The Mac Pro can be component upgraded, the iMac is an all-in-one, something that's becoming increasing popular in PC design btw.