I don't think within-family-car-sharing would be a HUGE deal, certainly. But there are a lot of situations in which I end up chauffeuring my fiancée or vice versa because one of us needs to get to point A, but the other needs the car for the day. And there's lots of little friction points in terms of lining up our schedules there. It's not a huge pain point, but my car would be noticeably more valuable to me if it handled these situations for itself.
It might not be wildly convenient, but it's even less convenient for me to drive my fiancée to where she needs to go and then drive back.
It might not be wildly convenient, but it's even less convenient for me to drive my fiancée to where she needs to go and then drive back.