All good until the owners of said cars want o use them after they've silently dumped their battery content to prop up the grid.
You really think that EV owners will allow their car's batteries to be entirely depleted in feeding back to the grid? Even if the inverter in the car would have a controller setting to feed back until exhausted, the owner won't use that setting. And ultimately frobbing in a chip that fakes the battery level to be lower than it actually is if such a 'deplete' setting would become mandated.
Also, using your car's batteries for grid support might be a possibility in the future, but AFAIK no EV has that option yet.