Is there evidence that it was actually used for playing games? The irregular placement of the divots makes it look like a kind of vandalism that was popular when I was in school, where people would surreptitiously use coins to grind divots in walls while waiting for class.

I don't have any definitive proof at hand, but it was mentioned in the brochure for the Baths of Caracalla. Here's a photo with a longer description: http://www.ipernity.com/doc/287951/48703648

I think this kind of stuff was relatively common throughout the Roman world.

It's more obviously a game board in this photo. The irregular placement increases the difficulty of the game.