Don't know if I should butt in because I work 90pc Airbus now with separate pistons for each brake system and no transfer.
But a few years ago I did nightstop checks on B757's and it was definitely worth transferring the fluid with the brakes.
Anyway no one has mentioned the brake accumulator. This is normally filled with the R hyd pressure, and returns into the R hyd tank. But after a nightstop, if the crews turn on the hyds from L to R then the Alt hyd system (L or C depending on aircraft) will fill up the accumulator. On the next nightstop it will empty into the R tank, and so quite a lot of fluid transfers. All Boeings seem to empty the accumulator pressure overnight. Perhaps they should check out the Airbus park brake system which lasts for days!