I have worked with a couple of wikis myself, mostly for scheduling: We used them for scheduling volunteers for an education program at my church, so that if there were changes necessary, individuals could make them on their own. That was a wiki on the church website, and the wiki was in the password-protected area of the site--there were only a couple of passwords for the community, depending on what position members held (deacon, member, etc.).
I can definitely see a wiki being useful in a library as a resource for the staff, similar to a binder of information at the front desk used to be. In a school setting, I can see it being a valuable tool for teachers to share resources, and especially for a librarian to build a resource that teachers can access as well as add to for the greater good.