What are the advantages or disadvantages to doing all push exercises one day (Chest, Tri, and maybe Shoulders) and all pull (back and biceps) rather than a back/chest combo like WBB1. Is either more efficient, or are they just a matter of opinion?

Q: Why do you have push/pull muscles grouped together? (From WBB2 FAQs)

A: The reason for this is that studies have shown that this method has a manipulative effect on the nervous system. This wil allow the lifter to get the most out of their workout. The scientific reasoning behind this is still somewhat of a mystery.

Does anyone know where these studies are? It seems like everywhere I look, people group push and pull together, but maybe that's just convention.