I don't think people arguing this have ever actually run the numbers and realize just how expensive rentals get. Every year my wife and I plan a week long road trip. If I price out this years trip from the midwest to Utah in a full size suv or truck (need 4wd and space for climbing/camping gear):

Base rental: $970

mileage fees: $350 (1400 miles at 25 cents a mile)

Tow fees: $180 (optional, but I like to bring my small teardrop so $20 a day for the tow package fees)

We're now close to $1500. At this price I can buy a truck, title it, insure it, and then sell it at a slight loss when I get home and still come out ahead of the rental. In just one trip.

*edit - formatting