As I understand it, the main goal of V2G is to help balance the load on the grid so that it doesn't need expensive power plants to come on-line to fulfill that momentary need. As a result, the utility doesn't need to invest maybe hundreds of millions of dollars for a plant that may only need to be used infrequently. This keeps the costs down for everyone. To be fair to an owner, the power provided by their battery should be replenished the same rate both ways at a minimum, and ideally, receive a credit for being a supplier during the peak needs.
In areas where there's a lot of solar or wind, if a vehicle could be charged when that power is in excess, that would help, too while letting the rest of the grid suppliers from fossil fuels maintain their consistent output...think of the batteries as a buffer. Balancing the needs of the grid versus the needs of the user so that the vehicle had enough charge to do what is needed when needed is part of the complication.
Just was reading about a new technique of building LiOn batteries on silicon wafers...faster recharge rates, less degradation, at least double the energy storage per pound, and less expensive. First applications may show up next year, but they've been demonstrated in utility storage situations already. Current design maxes out at around 100Kw. Battery degradation will become a non-factor in the future...building the infrastructure to take advantage of it should be starting now.
2011 535i x-drive GT, 2014 i3 BEV