The other point I forgot to make is that, during a storm, you could ground a lot of other non-essential electric vehicles, e.g. buses, and place their batteries

The other point I forgot to make is that, during a storm, you could ground a lot of other non-essential electric vehicles, e.g. buses, and place their batteries strategically throughout the city (at a bike station, or empty parking spots). Once the emergency is over, you're not stuck with a lot of excess inventory. That, again, assumes a lot: common batteries, etc.