Here our local park has 'front of the line' perks for Season Pass holders on certain days... but they are limited in #. Don't get there in the first hour or so.. and they're gone.
In a Disneyland or WDW scenario - where things are generally crowded - they will run out. If you give out so many you don't run out - the system falls down on itself just like Fastpass does. If you give out too many, your impact on waits will be greater, for both stand-by AND paid people.
The best thing to do is really make the price variable based on the expected crowd levels. This would match the price to the actual demand and supply. On low crowd days, you make the device cheap.. on high crowd days.. you make it more expensive. However, customers do not react to variable pricing all that well.