An interesting quote from the company's owner, from 2010...
http://tapmag.com/2010/09/15/will-that-be-paper-or-plastic/
"Paperless money is too hard to collect," said Dan Goins, owner of Kelly Coin Machine Company in Carroll, Iowa. "I like cash. The customers spend it and you have it in your hands."
(Of course, paperless transactions are also harder to hide from the taxman...)

I suspect most of the money in coin-op is from stuff like quarter-pushers, bartop games, and redemption crap. Even traditional arcade games, much less pinball, have fallen by the wayside on location. (There are still a couple of popular arcade video games... Golden Tee, Big Buck Hunter, and those Pacman/Galaga combos come to mind. They must be earning something, or they wouldn't be so common on site.)