Sorry for this kinda gamerbrained question.
The Xbox 360, Playstation 4, Xbox One, honestly most consoles after the Playstation and Saturn have shared memory pools. It allows flexibility in how much memory and VRAM developers want to assign, right? Why does the PS3 not have a shared 512MB pool of GDDR3? It caused all kinds of problems, most notably with Bethesda games.
Is it the Cell Broadband Engine needing the specialty XDR memory? Is it an artifact of the Nvidia RSX graphics chip being added late in development? Looking back I a)most wonder if the split memory was more of a problem than the Cell tbh.
Good post, it really is just different types of computers (although game consoles continue to get less and less interesting architecturally :( )
Also I started writing a response to your original post (interesting question tbh) that is quickly scaling out of control
