That's a good question, Dave... and one we gave a lot of though to, when we put RTS in both places.
We didn't go with advance tickets at the drive-in, because we couldn't find a way around the parking issue. First of all, everyone is waiting in line. Unless you had someone on the payroll, waiting for the advance ticket holders, you couldn't give them an "express line"... so no advantage there.
Unless the advance ticket(s) were purchased in one transaction, how would we know how many people would be in how many cars? You'd need to know, so you could make sure you had a parking space reserved.
Unless all patrons in one car had advance tickets, the whole group would have to go through the line... possibly not finding a good spot, and eliminating any advantage of buying in advance.
One of the big advantages of RTS is that it will print ONE ticket for all the occupants of the car, even though the total number of paid admissions is properly serialized for boxoffice reporting purposes. This makes it easy for us to verify admissions (read: look for sneakers). Advance ticketing would be challeging in this regard.
If you purchased an advance ticket, would you be arriving in a car?... a truck?... a semi-tractor? That would determine where you'd have to park. Chances are probably 100% that if we reserved a spot for you somewhere in the field, you wouldn't like where we put you... and we'd have to make sure there was an empty spot somewhere, so you'd have a place to park when you showed up. We'd also have to keep an eye on your spot, to make sure someone else didn't park there... which could also upset THAT customer (they did get there first, after all). Finding the best place that suits you seems to be a real drive-in tradition. Having us make that choice seemed unworkable and too much trouble.
So... we decided to leave it as it is... first come, first served.
We do make very good use of the RTS credit and gift card processing. With a broadband internet connection, processing time is about 3 seconds. A thermal printer spits the ticket and card receipt in about 2. That makes credit transactions comparable to cash, and we tend to see a lot of the larger transactions on plastic. That all helps us get customers through our 4 lines as quickly as it seems we can, so I'm pretty happy with the process right now.