The main reasons they gave for not offering V2grid:
(1) the car is designed to cycle weekly, more or less. Cycling daily is what the powerwall is designed for
(2) "If there's unexpected back driving of power from the car, it can create electricution risks do things that people aren't expecting"
(3) Regulatory issues with the car being a power source to a home.. Permits and regs. differ depending on jurisdiction
(4) unhappy family members when you want to take your car for a drive, but your car is the house's primary power source.
The thing is, none of these "reasons" is remotely valid, if the V2G is only used as an emergency backup in conjunction with a PowerWall.
(1) Emergency backups aren't used very often. Far less often than Supercharger use during roadtrips, which cycles the car battery every few hours, and Tesla doesn't seem to mind that.
(2) So use the car battery only to trickle charge the PowerWall; and not to power the home directly. Even if this requires two PowerWalls (trickle-charge one while powering the home from the other, then switch), it's still much better than needing ten PowerWalls to match the capacity of the car.
(3) Same answer: don't power the home directly, just trickle-charge the PowerWall. The permits and regs of powering the home from the PowerWall have obviously already been worked out.
(4) The PowerWall still works fine for a few hours while you're out driving around. There is no problem here.
The only legitimate reason I can think of to avoid V2G (oddly not mentioned on the call) is one of arbitrage, where users could go fill up at a SC for free, then come home and power their house from the car. Also, the associated wear and tear on the battery pack. Given that Tesla will likely implement pay-per-minute SC billing in the near future as an option for the Model 3, they could trivially also implement pay-per-minute V2G, at say $0.10/minute with a 6kW limit. (That works out to $1.00/kWh.) This would prevent arbitrage and makes it too expensive for daily use, but the whole purpose is only for emergencies, not daily use. And it would cover the cost of wear and tear on the battery, though vanishingly few users would use it often enough to make a significant difference. But in an emergency, it would be a wonderful option to have.[/QUOTE]