I can't believe this has so many upvotes! This is based on a silly assumption; aircraft are relatively more expensive than cars to buy, maintain, and use. So it is cheaper and affordable for more people if we time share them.
Cars are much cheaper, so we don't - the value of having a personal car ready right now is worth more to people than taking public transport. If aircraft were as cheap/convenient as cars to use/maintain/store/etc, then I'd have an aircraft parked up outside my house!
Driverless cars will have an enormous effect on society in lots of ways, but I highly doubt reducing the number of cars on the road will be one of them. In fact, I wouldn't be surprised if there are more cars on the roads, given that people who currently can't drive (because they're elderly/too young/disabled) will be enabled to.
Good discussion here: http://ftalphaville.ft.com/blog/2012/01/10/825201/the-ipay-p...