I mean, sure, it's more efficient, but it is also a way to make people spend more. Much easier to budget with cash. When you run out, you're done. With a card, it's harder to track.
that being said, Atlanta has really cheap concessions.
In my experience, it's not faster or more efficient. But it is a way to track exactly who's going to games, and it takes the "complicated math" look out of the hands of people who shouldn't be handling money (but still want a $15 minimum wage)