I'm not sure if I like your comparison to be honest.. not only is it not _just_ maintenance and filling the tank, if we stick with your picture, there is also things like buying the car, paying for it when no one uses it, insuring the car, repairing the car, general logistics of moving it around when someone has a one way rental, etc. - basically making it convenient for consumers to rent a car.

Looking at what "as a service" providers of open source software do though, that is taking it a step further, since they wrap the software in a layer that _smoothes_ out changes for the user. Going back to the rental company that would equate to the car manufacturer deciding the indicator needs to be on the right side of the steering wheel now and the rental company installing an adaptor so that it remains on the left for you, to keep the look and feel for the user the same.

Not a perfect comparison, but they never are :)