You can put 5L of gasoline into most cars and get decent enough range out of it allowing you to get out of a jam. 5L of gasoline also doesn't weigh very much and fits into a tiny little package.

The cells used in these vehicles all weigh hundreds of pounds. I suppose a standardized "spare" or "reserve" product could be made to wire into the car without disconnecting the permanently installed batteries, but it's going to be tough to create something small/light with enough juice to get the range required to drive the car to a service station.

I don't think that cars with limited-life batteries (less than 5 years) are going to take off. At least I hope they don't. Electricity as an energy source for vehicles is really only viable with new technology. Li-Ion is crap today in pretty much every common device, they'll be no less crap in a car. The eestor concept sounds extremely positive if a bit too good to be true. We'll see - but the playing field is wide open for new technology. It's an exciting time to be in these industries.