You've got to be kidding. The people who can afford multiple luxury cars aren't going to mess around using them as backup batteries just to save a few bucks on generators for their mansions.
It not a killer feature, granted. I would be willing to bet that the cost of the engineering to develop and support this feature as a default capability for the fleet of all vehicles would be less than the value of energy saved ammortized over the lifetime of all relevant vehicles.
