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.
This modular design what was made them able to do what they did - and imo what ps4 was the last iteration of for sony, with nintendo having the gamecube. The PS4 could do amazing things, but only like 12 programmers in the world was able to use it fully.
edit: Gc, not wii. Actually the gc was their first unified memory system. Sorry nintendo architects, my mistake