Sony have always liked dedicated video memory as both Vita and PS3 have separate memory pools this is easier with separate gpu and cpu chips. Also splitting into 2 main chips makes cooling easier so has thermal advantages.
You could have something like a fast exclusive memory pipeline to 4GB of ultrafast GDDR memory and perhaps 8GB of slower shared memory that both gpu and cpu could access. An 8 or 12 core cpu chip could have generous cache to optimise use to the 8GB of memory. Cache is slightly less important to the gpu as more likely to be unique data. The 8GB of main memoy would be positioned on the motherboard between the CPU and GPU. By providing dedicated pools of memory you could much increase overall memory bandwidth significantly over Scorpio. It helps make a high performance console in a compact case and doesn't necessarily make the console more expensive if it can make do with slower shared memory and only 4GB of ultrafast memory. By dedicating one IC to the gpu you can install more processing units into that I.C. so won't have to run it as fast to get decent performance allowing the console to stay cooler again. Consoles often have many of the same thermal issues as laptops and they often have discrete graphics to get decent performance, its no different to that really.
We have seen both ps4 and xbox one run their APU's quite slowly again because of thermal issues, much slower than 360 and PS3 at least for the cpu's, something like 1.6ghz compared to 3.2ghz. Splitting gpu and cpu's will help allow higher clock rates.
