When you get an engine failure the other bit that usually suffers at least a partial failure is the brain, it is easier to just remember to turn everything off (except master until last flap if electric flaps) rather than try to remember what to turn off and what to leave on
True.... but with that partial brain failure, will you remember which part of the two-part switch is the alternator, and which is the battery master? The effects of getting the wrong bit are worse than the marginal performance gain of getting the right bit. At least, that's my view.
FFF
------------