I find it odd to place the priority on having people in the car rather than people at the destination. Wouldn't you rather socialize with people of your choice at your party/book club/sewing group/whatever than with the weird and awkward people who happen to be on the bus? I know I would. Especially if the former situation is more efficient, so I get to go more places.
Put another way, the city I live in (Denver) has what is essentially a taxi service for disabled folks (http://www3.rtd-denver.com/elbert/accessARide). I know a couple of folks in wheel chairs -- and they use it a lot! I've never once heard them say they felt isolated because they weren't standard bus routes full of people. On the contrary, not being able to go where their friends were going is what would be isolating.
People tend to have fewer friends and become less able as they get older, and even incidental interaction is still vastly better than none. If we had a way to make sure disabled people always have company and no one is left behind, I might agree to the idea of taking them off the busses and giving the (semi)private rides. But we shouldn't assume they all have friends to go see just because you always had such friends.
And then, spatial mobility is also very important. For example, I felt absolutely trapped by my relatively benign broken foot problem.
Put another way, the city I live in (Denver) has what is essentially a taxi service for disabled folks (http://www3.rtd-denver.com/elbert/accessARide). I know a couple of folks in wheel chairs -- and they use it a lot! I've never once heard them say they felt isolated because they weren't standard bus routes full of people. On the contrary, not being able to go where their friends were going is what would be isolating.