Look at how many chargers are nearby and how far you'd have to drive in a typical rolling black-out to charge. I bet it isn't that much worse than how far you'd have to drive a gas car to refuel, as those stations tend not to have reliable backup power either.

On the US southeastern coast, the big concern is hurricanes. Gas stations are far from 100% dependable in regional emergencies both before and immediately after the event. Electricity tends to only be a problem afterward.