They make things safer because they don't drive drunk.
Can an hunter ride share the pacifica out to the forest with his/her hunting gear? How about bicyclists? People with kids know about activity shuffling... I am not sure that sharing will work for my families schedule.
Driving drunk isn't the bulk of the accidents I go to on a regular basis. If we could just eliminate texting while driving, we could see a significant drop in accidents.
Furthermore, a lot of people use their cars at the same time of day. It's also the case that a lot of auto depreciation is mileage-based rather than time-based (especially in non-snowy/salty environments).
The questions others raise about the maintenance/update requirements for these sorts of vehicles are interesting ones.
My guess is that, like Zipcar and Uber, they will make a difference at the margins but won't cause a wholesale shift. But who knows? Especially if, as I believe, this happens over a number of decades with demographic shifts happening in parallel.
Easy: many households now have two (or more cars), but self-driving cars will allow many of them to reduce to a single car. One people carrier for family outings, on-demand robot taxis for commutes and quick errands. Maybe not a total revolution, but a 50% reduction in household car ownership would be a pretty big deal (it'd make neighbourhood parking a lot easier for a start).
I mean, let's say I use it for a total of 12,000 miles per year. that's 10% of the time. If I rent it out for an additional 50% of the total time, that's going to hit 72k miles in a year! For the average person they are going to have to replace cars ridiculously fast, and increase, not lessen, the burden of car ownership.