I've actually been through multiple evacuations before, and practically every mile of energy in our vehicles counted. One time one of our cars literally ran out gas rolling into the first gas station with fuel after leaving with a full tank. When major storms are coming my cars stay topped off and don't get touched except for very purposeful reasons.
And given the fact it might be harder to find a working and available charger while leaving in an emergency I wouldn't want to rely on finding one along the way to wherever I'm going.