I'm not exactly thrilled at the ingenuity here either but lots of people leave their car at the airport for a week, two weeks, sometimes more in long-term parking. If you could wipe out that cost by letting someone use your car, why wouldn't you? Or if you want to avoid the usual rental rigamarole, why wouldn't you? Seems pretty sensible to me, if not particularly original.

It's weird that your thoughts jumped straight to an 'investment car.' Many people only use their one car occasionally, opening it up for rental, and two-car households can save or even make money by renting out one when it's not necessary to use both.