Well, being that Intel showed up so late to the x86-64 game they really didn't have a choice. The initial iteration of Yamhill was disabled in Prescott due to it's x86-64 incompatibilities
Well that's your opinion, given the speed of microsoft and thier OS, I'd say they showed up right on time and why not use AMD's extensions, didn't AMD use Intel's? alls fair and heck it was good management. :)