I've noticed two phenomenon since I moved to the St. Louis area from Los Angeles: there are more churches here, and there are far more billboards advertising churches. These must be related.
Perhaps in Los Angeles churches don't think it's worth advertising because they don't think the ads will have much of an effect. Maybe churches don't think there are many potential attendees who don't already attend. However, in St. Louis a much higher proportion of people attend church, and it seems even more unlikely that there are unreached potential attendees.
Perhaps churches in Los Angeles don't have the money to rent billboards, despite a desire to do so? I doubt it. There are plenty of large churches in Los Angeles with plenty of money.
What seems most likely to me is that churches in St. Louis aren't trying to non-attenders -- the billboards are generally cheesy, and I think Los Angeles churches are right in thinking that billboards wouldn't attract unbelievers. No, the reason that I think St. Louis churches put up billboards is because they're trying to get attenders to switch churches. With so many churches, competition must be fierce for attenders, and they figure the population is dense enough with Christians that billboards are a cost-effective method to convince some to switch their attendance.
Seeing as how none of the signs I've seen have advertized any Biblical virtues or encouraged spiritual growth, but rather mere "happiness Christianity", it seems that the churches that are deepest into the competition are also the churches with the least real spirituality. Overall, I don't think the billboards speak well of the churches that put them up, especially nonsense like the Methodists' current "If you can wish, you can believe" campaign.
This is a result of the "we're selling a Jesus experience" mentality that many evangelical churches have (even very small ones). I think the remedy is for churches to recognize that they're not selling a product and stop looking to 'church growth experts' for advice.
My friend is an elder at a very small conservative, reformed Presbyterian church here in Charlotte. He made the comment in a session meeting once that "we're not selling a product here" & at first they agreed but then later came back with .... "well you know... in a way we are selling a product" Yea in the same way you're going to end up on a cheesy billboard on the interstate.
You're right about the ads not being directed towards unbelievers. I think some of them however are geared towards "lapsed" Christians who have been turned off for whatever reasons by previous churches.