The motherboard "knows more" than the PSU though, it should be able to better deal with these idle power situations by shutting down things that aren't used any

The motherboard "knows more" than the PSU though, it should be able to better deal with these idle power situations by shutting down things that aren't used anymore and powering them back up when needed. An external PSU would have tighter constraints regarding power availability and wouldn't be able to optimize the power profile quite as aggressively since they don't really know exactly what they're driving.