Most batteries have a life limited by the charge/discharge cycle. So using the cars' batteries for other purposes shortens their lives, and they are expensive enough as it is. Good thinking, though!
The only battery that I've come across that is infinite in its charge/discharge life is the vanadium redox - but that has cost and space issues. I got a quote of $50k for my house - but I don't think they were really interested, it took a lot of hassling to get just the quote.
A fuel cell running on hydrogen, and using solar/wind to electrolyse water to produce the H2 for use at night? Cost effective solar cells needed, but that will come in time.
