The catch is what's made public is the 12+ numbers, which really don't mean a lot. You really have to dig deeper to come up with something meaningful. Of course, to do that, you have to subscribe to the service and then you are more or less sworn to secrecy.
Also, keep in mind that the data Arbitron releases is really fairly old information by the time you read it. You'd think in the days of computers and instant communications, these reports would be much timelier. Releasing the "Spring Book" in August seems a little like selling last weeks Sunday paper. A lot can happen between the close of the rating period and the release of the information.