Would make a for huge mess in the marketplace, all practical problems with manufacturing aside.
Joe Schmoe wants a basic $300 computer for checking his mail and stuff, so what does he get?
A Celeron with the weakest possible GPU on-die?
Then his kids wanna play WoW all of a sudden, he now has to buy a new CGPU(or whatever they'd be called) with a decent graphics core, so he gets a Core Medium.
Then your average AT'er wants his new uber-l337 ultra gaming rig, so he uses his daddy's VISA to buy the best CGPU out there, which happens to be a Core Extreme.
Me on the other hand, I enjoy the occasional game, so I might want something in between the Core Medium and Core Extreme, say the Core Prettygood.
In the end, you'd end up with a boatload of different CPU's, not only of different speeds but with completely different cores, manufacturing that many different cores would make prohibitively expensive.
Just looking at nVidia, we have what? 4-5 different price/performance points, ranging from the stuff that's barely any better than integrated graphics to the $700 SLI-on-a-board cards, and those in turn can be SLI'd, so you have yet another price point.
Combining these with the already existing price points for CPU's, different speeds, cache configurations, dual/single/eventually quad core, etc, you'd have so many processor lines and price points that even the hard core enthusiasts would have trouble keeping up, Joe Schmoe can pretty much just forget about it.