I've seen this quite a lot, especially in NZ & AU where it works very well. Unfortunately it's not a universal panacea. I've been stuck behind a line of 10+ cars when the prepaid exit ticket would not work due to time out.

Once you pay you have a grace period of about 15 minutes to vamoose which is fine for a small parking area. If you had the same system in say LAX, you'd pay, cross the street, wait for one of the slowest elevators in creation, pop up to the 5th floor, find your car maybe 300' away, drive down the circular ramps for five stories perhaps behind a line of cars for people who've just met the three 747s which have just arrived. Bonus points if you have an elderly traveler.

I also occasionally leave my car at the regular airport parking for 3-7 day trips and leave the parking ticket in the ashtray. With the pay at the terminal system I'd have to take it with me and hope it doesn't get lost or misplaced.