AAA estimates that it costs roughly 40 to 97 cents per mile to own a car (depending on the type of car and how much you drive)[1] so I'm having a hard time seeing that a lot of people will let random strangers abuse their cars for 5-20 cents per mile. Because let's face it, nobody treats rental cars as well as their own cars.

[1] http://exchange.aaa.com/wp-content/uploads/2013/04/your-driv...