People don't seem to understand that the battery chemistries optimized for cars is not the same battery chemistries optimized for grid storage though. You'll reduce the lifetime of your car substantially by cycling it every night.
Degradation of lithium batteries is also not constant. It depends on several factors such as C rate, heat, and voltage. All of these factors would be far less during load sharing than just driving your car.
And keep in mind most people wouldn’t want to share their entire batteries. They would want to save most for themselves. If they only offered 10% of their capacity for load sharing, then the shared usage would be far less than a full cycle.