Earlier quoted context omitted.
If the power market is fully 'open', then yes, it will happen. All it takes is for a car scrap dealer to realise that for every EV battery he grabs out and hooks up to the grid he'll earn $200/year. Before long, every scrap dealer will be doing it to every car. However, I suspect that small players will be barred from the electricity markets via mountains of bureaucracy and certification, so that might not happen.
That doesn't really scale. The power grid needs to be reliable, and there needs to be some idea of how much capacity you actually have. If people simply hook up an unknown number of batteries of unknown capacities in unknown spaces, how can you rely on the resulting power grid? How can you plan extensions? Not to mention high capacity batteries are huge fire risks, especially worn out high capacity batteries. Can you…
At scale, everything becomes more predictable, because individual failures no longer have much impact.
Battery operators also will configure their systems to make money, so they will in fact have a big incentive to buy power when it's cheap and sell when expensive, so they will form a strong stabilising force. (unlike say solar, where operators don't track the price and just inject whatever power they produce, even if the spot price of electricity might occasionally go negative, and when it is negative, there is unconstrained overproduction)