Q:

Connecting Raspberry Pi inline with ATX power supply standby

I am looking to have a Raspberry Pi in my desktop computer to run some auxiliary functions.  There is a lot of information about using ATX power supplies with the Pi, but I haven't been able to nail down my exact situation. 
I would like to power the Pi using the standby rail on the PSU, which supplies up to 5v/3A. There is a lot of info about this and I'm confident this would work, but the idea is to still have that power supply running the main computer.  The Pi would be always on regardless of the main machine.  To connect the Pi I would splice connectors into the 24 pin standby and ground wires which will go to the Pi. 
I still have some things I'm not sure about with this configuration though. 

Is there anything wrong with connecting the Pi in parallel to the motherboard as described above? 
Will the standby rail continue to supply power when the main PSU rails are on (the computer is turned on)? 
While the computer is off, will plugging other things into the USB ports that are on the standby rail cut or lower power to the Pi? 

Past that, the first use for the "daughter" Pi I want to implement is to use it as a software power switch.  In my mind, I could just apply 5v to the power switch pin on the motherboard with a single wire, and it would act as the completed momentary switch circuit for power on because the Pi and Motherboard share a common ground.  Is my understanding wrong on this? 

A:

There are no problems doing this.  Read this recent answer: Is it possible to power an orange pi/raspberry pi directly from a computer's power source? 
I run a configuration with two Raspberry Pi' using the +5SB rail.   works like a champ.    

Is there anything wrong with connecting the Pi in parallel to the motherboard as described above? 
None at all, though I'd suggest you buy a cable to graft the power connection in rather than cut your ATX power cable. 
Will the standby rail continue to supply power when the main PSU rails are on (the computer is turned on)? 
Yes, this rail is powered on whenever the PC is plugged into AC power.    
While the computer is off, will plugging other things into the USB ports that are on the standby rail cut or lower power to the Pi? 
No, providing you don't exceed the power rating on the +5SB in total (typically 3 A)

