At our airbnb in Santa Cruz, I estimate that 30% of our guests are out of state and would love to have a car waiting for them to use when they arrive. This is the main reason that care sharing held a certain appeal to me - a marriage made in heaven.
One challenge that this poses is the added cost for a guest to get from San Jose International or San Francisco International airport to Santa Cruz. The guest can find their own transportation (an Uber is about $45) or I need to come up with a strategy for leaving the car at an airport for pickup. The latter is probably more problematic than I care to deal with.
From the guest's perspective, they now have a one stop shop to communicate with regarding both their car rental and airbnb host. The added cost for the car rental in our case would be about 20% over the daily airbnb rental (~$40 USD). We would most likely offer a 15% discount for the car rental for airbnb guests.
Since the two rentals are completely separate, there won't be any benefit from Airbnb's insurance policy. If/when Airbnb also offers a car sharing option, that will help streamline my operation.