Huh? Why are you including CPU, this isn't the Cell, it doesn't help with graphics, only drives logic and can become a bottleneck but can't magically push pixels with the GPU.
Eh? I'm pretty sure the CPU aided in triangle setup and culling and other graphics tasks. You could for example take a buffer break it into tiles and determine how many lights affect each tile before sending it back to the GPU to be shaded. The GPU was able to read L2 cache, afterall.
Sure on Cell if you had a G-buffer you would just shade on the SPUs, but implying Xenon was only good for "logic" is incorrect.