Why is there the requirement for switching on the Ignition in an Emergency Descent? Obviously airmanship wise it's never going to be a bad idea but anyone with more than that to add?
I suspect it is simply a risk mitigation measure. An emergency descent is a busy time and descent through icing is likely and through unknown or unconsidered levels of precipitation is quite possible too.
Given the possible slow reaction by the crew due to workload and those potential changes in the outside environment, I think it should be judged an essential safety measure.