I should clarify: the lucrative buyback programs that the rental agencies used to rely on to supply them with a virtually unlimited number of cheap cars without tying up lines of credit have mostly dried up. Some remain, but with the U.S. auto manufacturer bankruptcies and subsequent labor agreement restructurings, production has tightened up.

Nowadays, rental companies have to buy most of their cars ("risk" cars). The best deals are coming from the Far East auto manufacturers, so you're seeing a lot more Japanese and Korean cars in the fleet mixes. Because these cars are bought and financed instead of leased on short-term buyback programs, you're seeing cars stay in the fleet longer (50,000 miles is the new 25,000 miles). Purchasing and financing the cars also ties up lines of credit, meaning that there are a finite number of cars the banks will let the rental agencies buy.

All of that means that fleet costs are up, and intense competition among the value brands has driven the prices in many markets to rock-bottom prices. Thus, unbundling (which has always been prevalent in the rental industry) is an attractive way to help make up the difference.