Presumably as we use more renewable energy, we'll need more temporary storage. Buffering out day/night cycles from solar generation, for instance, would be a daily occurrence not a rare event. $9/kwh might be reasonable in an emergency, but even 18 cents/kwh seems pretty high for daily use.
For you though that cost is just part of the normal depreciation of using the vehicle. Allowing the utility to discharge your battery isn't part of the normal utilization for yourself so the cost should get included. We also don't usually think or talk about how much depreciation using a particular gallon of gas costs you so it muddies comparisons.
the "real" time-based prices have much more variability, if you have time-based prices you can buy for a lot less and sell for a lot more during peak. That difference can easily be double that 18 cents/kWh.
That is interesting, that means for home-charging your electric cost (per mile) is about equal to the battery degradation cost.
Thats 2x cost than most think.