I like these ideas, but they call come at the cost of the availability of the car, and I don't know how often people will actually do them. Let's say you send your car off, and it does an errand for you and is due back at 11:30, so you can take it out for lunch at noon. Now the car gets stuck in traffic, or the weather turns bad, and the car can't return until 12:15. That's frustrating. Likewise, say you send it off to drive around until you need it at 5. However, everyone else is also doing that, and there's a huge traffic jam at the entrance to the financial district, and your car doesn't actually show up until 5:30, causing you to miss you dinner reservations.

I've owned an unreliable car, that didn't start every time, and it totally sucked.