That seems like a fantastic idea. This way, the car's internal battery can be smaller and lighter for day-to-day use (and therefore more efficient), and whenever longer range is needed you can just rent a fully-charged tow-a-battery, which you can swap out like propane tanks, or recharge yourself at home. These tow-a-batteries would work with any electric car with standard secure hook-ups.

Since these batteries would be rather expensive, I imagine that you would have to pay a refundable deposit to the tow-a-battery network (Shell, Esso, etc.) that covers the full cost of the tow-a-battery in order to rent one.

The biggest down-side is that a rear-end crash could be rather dangerous. I don't see a way around that without making the tow-a-battery a vehicle in itself.