We had a 2-line readout at our indoor boxoffice. It didn't really give the customer much more than the final amount, but maybe that was enough. At least you didn't have to repeat yourself if the customer forgot.
At our drive-in, we don't have them, and I'm not sure they'd be much value in a fast-paced line.
As for concessions... in the end, simple displays are only going to give you totals anyway. That might be a crutch for customer and cashier alike... one less opportunity for personal communication.
In my experience, I'm not convinced it matters one way or the other... adding them here would be pretty low on my priority list. Your experience may differ!