In this article by Paula Webb, YouTube is presented as one solution to libraries for a way to tutor patrons on how to use the library's resources, or as a creative way to show patrons the resources available at their local library.
While I'm not completely adverse to the concept of using YouTube in such a fashion, this method of posting a library instructional video on the library's web page would only be useful to patrons who had Internet access. I also believe most patron's first experience with the library does not occur online, and that usually the library web page is used most often to access the catalogue database.
However, I think the best use of a YouTube video for my local library would be to show all of the resources available to the patron as a sort of online commercial for the library.