EEngr:
Same thing goes for the main battery. Look at its braking function. That is a non-time dependant requirement. No matter how long the flight, you need the same braking power at the end. If the probability of failure during a 330 minute diversion is intolerable, then a factor of ten isn't going to buy you more safety.
I have not seen it in writing, but the discussion in the Webex seemed to imply that for a plane which is landing the brakes can be powered from the RAT or some other short duration battery, and that the ability to brake using the main battery is only needed when the aircraft is being towed on the ground with neither engines nor APU running.
The APU battery is more significant, since it appears that, regardless of other power sources, loss of the APU battery will render the APU inoperable. I would imagine that taking off with the APU battery inop would not be allowed.