All these items you mentioned (ailerons,elevatirs,rudder) have redundancy,if one system is inop (A or B) then the other will still provide sufficient volume/pressure to operate them,if not then an secondary source is used(ie TE flaps electrical motor).
Why would you want a line of fluid open with no or too little fluid/pressure supplying it?
It could be simply that under a certain pressure,the above flight control cannot simply be operated.For example,If you lose engine hydr pressure only the electric will take the load but may be struggling to cope with the required pressure demand for the flight controls it operated and low pressure may illuminate.If you lose both pumps then they cant be operated.
A and B hydraulics dont mix.
The standby rudder is powered when the switch is in Stby rudder,useful when you plan on landing in windy conditions.
Mr Boeing says the PTU is there to provide volume of hydraulic fluid to allow operation of the LEFlaps/autoslats/slats at the "normal rate" when system B is lost; the electric pump only provides 1/6th of the volume of a mechanical pump hence the PTU to use the A system mechanical pump if required.
The PTU will not work if you have lost System B fluid...only if its pressure has dropped under a specific value (2350psi).
It then uses A pressure but B fluid.