I think it has to do with the caudal of hydraulic liquid available in the system. You can have same pressure, but with lesser quantity some systems (heavy users), might not work completely fine.
I was a fan of switching in the yellow electric hydraulic pump when on the APU during an emergency landin with both engines flame out (only in the sim!), until one day the checker asked an obvious question:
- "Why you doing that Iggy?"
- "For the benefit of having the yellow hyd system working in a situation when control of the airplane and braking power is needed the most?"
- "Don't you think that, if doing that was somehow helpful, they would have written down it on the list?"
- "But, they are French, and we know better than them in this airline, right?"
- ...
- "Right?"
- "Look, if it is an obvious thing to do, but it is not written, there must be a reason for it. And in this case, the reason is that the caudal (the amount of liquid, I'm not sure if you know the word caudal, Iggy), that the electric pump can deliver is around four times less than the caudal delivered by the EDP. Understood?"
- "Yes Sir!"
- "Ok, now do me 50 push ups, you are fat".
So, not being in the book means has more to do with people like me being allowed in the cockpit more than being esoteric.