A valid CC is no longer required if you are Preferred. I have rented many cars from their Signature Series all over the states on a VISA Debit card.

At any US airport, even some European you simply show your license; either at the Preferred desk or at many locations, a billboard has your name and a stall number on it and you go straight to your stall where your car is parked, pick the keys, show your license to the attendant and you're out.

Many US locations has "Select & Go" where you can choose to change, upgrade or keep your current car. Works excellent, specially at MIA.