That is the strangest advice to not reserve. Reserving means locking in a rate, while showing up the day of could mean anything from the rates shooting up to no availability. I have had to wait for a car before, but if I showed up without a reservation I don't know which place has the best prices and I might have to wait behind all the people who have reserved.
I rarely run into a case where I have to wait long for a car, but that affects everyone. I've also had no availability of my reserved vehicle type and have gotten a different car at the same price.
I have had cases where I made a reservation less than 24 hours before picking up a vehicle, but I didn't strictly need a car. I actually was arriving too late by bus and it was too late to cancel my original reservation. I found a rental agency for the next day that had plenty of availability, and the one-day rental was even cheaper than my original reservation.
One deal is that the majority of car rental reservations aren't prepaid nor guaranteed by credit card. You can literally fail to show up without penalty. I've had at least one time when the car was optional and I just went up to the counter and said I didn't need it. If you can get a better price or better availability, you can easily walk away from a previous reservation without any penalty. Once I had three reservations active at the same time, although I did cancel all but one to be fair to the rental agencies. I thought maybe there was a risk that one rental agency might cancel two simultaneous reservations, but I guess it would have been possible for me to rent two cars at the same time. At least in California, someone is automatically allowed to drive a rental car if a spouse has rented it, provided standard age and licensing requirements are met. The spouse isn't required to be listed on the rental paperwork. So renting two cars isn't something that's out of question for being impractical since a spouse could just take the second vehicle.