They would only improve space utilization in terms of parking. You still need the roadways, which are more problematic in terms of space, maintenance, and walkability of cities. Then you also need to account for the increased production and maintenance costs associated with replacing parts like this and drivetrain components on vehicles that are active for most of the day.

Increasing the up-time isn't necessarily a good thing. That just increases energy usage and wear on wearable parts like tires, drive trains, etc. Cars aren't just inefficient because of down time. Hell parking is only one of their space considerations. Roadways are a significantly more important space consideration in which low- occupant cars are highly inefficient. This is because roadways replace walkable areas for pedestrian traffic.