As an exercise for the class, we were instructed to look at six examples of wikis that are being used in libraries. They are being used for subject guides, electronic resource guides, staff information, reference guides and OCLC's user comments system (to be released apparently in September, according to their website).
After looking at the examples, I would conclude that wikis are useful as a way to collect, organize, present and update particular information. The UConn staff wiki was particularly well organized, and obviously useful (for the staff, that is). I also liked the St. Joseph County Pulic Library's Subject Guide wiki which was very well organized, frequently updated and easy to navigate.
One of the features of a wiki is the possibility for user interaction- something that seemed to be missing from all of these examples. I think the chief value is if multiple people in an organization need to contribute material, a wiki is a convenient way to collaborate.