It is an appealing idea to use the car as a battery store, however, there are a few practical issues.
1. When using the car as a battery bank, we eventually increase the charge / discharge cycles of the battery, leading to quicker need for replacement. An external solar battery is cheaper, considering it need not be miniaturized.
2. When not in use, I'd prefer the car is charged, so that I can use it as and when immediately required, say an emergency. So, using power from the car, is not appealing, to me atleast.
3. Grid is prone to brownout in India, however, most cities and towns are experiencing it less and less and these are the major markets for EVs.