As the name implies, the ATX12VO standard is essentially about removing the other voltage rails that currently exist in the ATX PSU standard. The idea is that by providing one single base voltage ...

Nope, take a look at the ATX12VO spec. It's stupid and doesn't reflect how modern machines work. The main motherboard connector only has 3 pins for +12V (to standard ATX's 2), but PSUs above 450W ...