Tom - I was thinking of the case of the EasyJet A/C when they lost AC1, and consequently AC + DC ESS.
Haven't got the FCOM open in front of me, but depending on the failure it may be a fault elsewhere on the network. Dropping the RAT supplies the ESS network through the ESS TR, and on top of that sheds some of the bus anyway.
So it's possible that if the fault lies elsewhere, or only on one part of the bus, dropping the RAT may resupply the DC ESS bus. Or am I talking out of my arse? (Very possible!)