Well, there's absolutely nothing wrong with it, if you know for a fact that this billboard/Yellow Pages double whammy is actually growing your competitors' law practices. It's amazing how many lawyers will take a “monkey see, monkey do” approach to their marketing efforts, investing no more effort in their promotional campaigns than looking out the window (or turning on the TV) and seeing what their competitors are doing.
But unless your competition has been doubling in size every year—while your law firm has dwindled down to you and your secretary—you may want to rethink whether imitation is really the sincerest form of flattery.
Let's look at it this way: perhaps, years ago, there was a visionary lawyer in your area who saw that billboards were the way to go, since they reach a captive audience of people stuck in rush-hour traffic. For a time, the billboards worked—and then a competing firm decided to buy space on its own billboards, and the firm after that, on and on down the line until it's your turn to commit your hard-earned money. The law of diminishing returns dictates that billboards will no longer be an effective advertising medium, since literally everyone is buying billboards—at which point motorists will fail to differentiate one billboard from another—and one law firm from another—and the medium will be completely played out.
Copying what someone else is doing—without knowing whether it’s truly successful—isn’t a law firm marketing strategy. It’s a way of washing your hands of the task, and refusing to take responsibility for the success of your own career. If you believe you’re a better lawyer than that, then you owe it to yourself to make a honest effort.