PC World has some more details about the benefits of ATX12VO. Current multi-rail (5V, 3.3V, 12V, -12V, 5VSB) power supplies aren't very efficient at low loads because the PSU sends very low current to all voltage rails. This results in an energy efficiency of just 50 to 60 percent in idle mode. By switching to a single rail design, ATX12VO can achieve 75 percent efficiency at the same DC load levels.
And while efficiency will go up, the cost of a power supply will go down, while the cost of a motherboard will go slightly up.
The downside of ATX12VO is that it's not compatible with current PSUs and motherboards. Motherboard makers seem largely optimistic about the move:
Motherboard vendors whom PCWorld tapped for comment seemed largely optimistic about ATX12VO. One told PCWorld the move would let a motherboard better manage the power sequence during boot-ups, which can get sticky when a non-standard PSU is used. With the motherboard in control of all three rails, it can better monitor and calculate power consumption, and it can reduce the risk from abnormal PSU power spikes.ATXV12VO will first be used for OEM systems. It's unknown when or if we can expect the first DIY products.