Wawa does this, and I love it. Granted, it's not a restaurant, but ordering from them is very easy and quick, and I prefer it to having to give someone my order. I can still talk to the people behind the counter, but without the headache of ordering.

In a restaurant, you'd still need a wait staff, but instead of placing the order, they can work to address your needs and better serve customers. While I imagine places would cut down on wait staff, I also think the service would improve.