As an electric car owner, do I want to give my limited lithium battery charge cycle lifespan back to the network? This storage isn't "free" to the vehicle owner - charging and feeding back will contribute to the total charge cycles on your very expensive battery pack, which of course reduces range over time. I'd need to be heavily convinced by the economic arguments outweighing the wear on a very expensive vehicle component before I'd consider this.

When you come to resell an electric car, the condition of the battery is going to be significant in resale value too.

There would presumably be a zip code lottery element here too - would some areas of a given country make much greater demands of resident's privately owned car batteries than others?