The MEL states that you must have at least one centre pump serviceable, so for the purposes of this conversation I assume they are both lost during flight.
The arming requirements for RAT deployment are air mode, aircraft above 80kts and both engines out.
As stated by Expedite climb you can fly quite merrily on the L/H and R/H Hyd systems.
I can only assume that along with the extra fuel burn, the RAT is not continously rated above the the aircrafts ETOPS certification.
If the L/H and R/H systems were to fail together your only means of having a 'good day' is the Centre system RAT and you would not want to jeopardise its integrity by using it when technically not required.
Regards