I can see some minor value as a backup source of power in case of a brief power failure at home if that's where you charge it, but off the top of my head, I see a much bigger downside.
1. Electric vehicles already have a limited range. so If you depend on the vehicle and need it while discharging power, you've either reduced the useable range even more or must wait til charging it back up.
2. Since you must charge up the car to use it, If you discharge it trying to save money by returning electicity to the grid, what happens when you want to use the car? Uh, You pay to charge it up again.
3. Unfortunately the length of outages are unpredictable. If you tie up your car powering your home during a PROLONGED outage, your car is pretty much dead as you have no way to recharge it until power is restored. But had you left it charged, you'd still be able to drive it during the outage. then you can drive it to a place there is power to recharge.
4. The more often you discharge/charge the batteries, the more you shorten their life and the sooner you will need to cough up the several thousand dollars to replace them. I'm sure that would exceed what you save discharging it to the grid.