From the FCOM schematics and description (rather than the simplified representation on the ECAM ELEC page) it looks to me like the DC ESS bus is powered directly from Bat 2 when on battery power only, and doesn't require the DC BAT bus. If that's the case I'm still not quite clear on the reason for all the on-ground switching, specifically:
1. What is the advantage of restoring the DC BAT bus <100kt?
2. Why is is AC ESS bus shed <50kt? One significant effect is the loss of ECAM, which might just be useful. The battery drain issue doesn't seem to be the answer, because the schematic shows the inverter powered exclusively by Bat 1 (so why not just keep the AC power for as long as that battery lasts - it wouldn't affect the comms equipment being supplied by Bat 2).
There's logic in it all somewhere, I'm sure!