Yeah we don´t know wich part the eDRAM plays in the memory system.
My guess is that its plays a very important part in adressing the badwidth/latency issues.
Its also the memory controllers and the proximity of the GPU to the CPU.
Here is how I see it
What's more powerful, a plane or a bike?
Would you take a plane to go visit your friend across the street?
No, only if he or she was on another continent.
The issue with video cards for PCs is that the GPU is at a distance from the CPU.
The further away it is, the more latency you will encounter. So to make the CPU and GPU
communicate as fast as possible, video cards need to have fast ram, large bandwidths and strong GPUs. Just like an airplane needs a large runway vs a small street for a bike; and a powerful engine to cross long distances at a super speeds compared to a bike.
What Nintendo did was to basically put the GPU right next to the CPU. The minute they did that, they didn't need an airplane anymore. They could basically get away with using a bike to get the same results. In return, such R&D meant they could make a less expensive console and pass those saving to the customer. Nintendo did not go cheap with producing the WiiU, they went smart.
If Nintendo kept the same architecture as they have done with their previous consoles. Then it would be like taking a bike to visit your friend on another continent.
My analogy might be completely wrong, but thats how I understand it.