Looks like the policies may be changing and we won't be able to make multiple reservations without canceling ahead of time.

"Global distribution system operators are preparing by December to take credit card information for bookings with Avis Budget Group in the event the car rental firm decides to apply a new policy for no-show fees."
and
"So, while for some locations and time periods we may choose to impose a no-show fee, for others we may choose not to. Our new capability will align the car rental industry with similar approaches in use by airlines, hotels and cruise lines. It is a long-overdue change."

ViewfromtheWing Blog
Management.travel Blog

If they decide to start charging for no-shows, then they should be held to guarantees of car type availability. The main reason I make multiple reservations is that I've been burned by no car availability or car type ( e.g. SUV in a ski area).