The first adjacent pirates will lower the value of 94.7.
The first adjacent pirates will only lower the bid value of WFME if the potential buyers drive around to check the signal and take note of the problem.
The kinds of neighborhoods where the pirate signals are the strongest are also the kinds of neighborhoods where the fancy lawyers, consulting engineers, bankers, and broadcast executives never go. These guys would also probably be shocked that these illegal signals have been out there for years with regulating and policing authorities doing very little about it. They would also probably assume that if there was serious audience size damage that the FCC would deal with it.
But the pirates don't just interfere with people trying to listen to 94.7, they cause interference somewhere for just about every licensed station on the band, and nobody, including the major commercial station owners, seems to care that much except radio types on internet boards like this. In the end, the pirate problem likely will not impact the FME license sale price, and the pirate problem will continue until some "mover and shaker" with some real juice can't hear his or her favorite station because of a pirate, has a hissy fit, and calls the right people to demand they do something about the problem. But I wouldn't hold my breath waiting for that to happen anytime soon.