I will confess to backup reservations, but only in one very specific circumstance.
On rare occasions, $ has a very good rate, one that can not be ignored. But, I once had a $ reservation and when I got to the airport they were out of cars. The entire city was sold out. I ended up having to take a cab and $ refunded all my cab fare.
Now, if I fall for the $ low ball rate, I will back up at National or Enterprise.