There's a lot of truth to that.
YP is used primarily by people who are new to a given area. This includes folks who are passing through. Hence the plethora of restaurants and lawyers that buy huge YP display ads. They're catering to customers who don't have WOM referrals for services.
People who are new to the area and those who are passing through aren't likely to need roofers or DJ companies. There are exceptions, of course... but in general, the YP's target audience (those new to an area) doesn't have a need for DJ services.
