After viewing all of the sample Wikis I see many ways to use them.
The first one we viewed was more of a communication wiki that was a tool to get information out to parents.
The second wiki that was used in the English class was a coloaborative wiki, but she gave the parents information about how the wiki will be used, how to contact her, and how assignments cannot be late.
The last one was both an instructional and collaborative. Assignments were poster to do, but he had great websites to help his students.