grow your wiki
The Clean Permissions pattern is to set up an authorization system that is easy for wanted users to use and understand, while minimizing the amount of trashing and other forms of revertable grief from the unwanted.
Post a clear explanation of the permission model prominently on the wiki. For each action requiring authorization, make sure a clear explanation of the authorization criteria is easily accessible.
The original c2.com wiki has a clear and detailed discussion of its Permission Granted scheme and why it works for them.
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I think this is exceptionally good for a public wiki, it keeps up the transparency of the wiki.
However in a corporate environment i think it may be hard to explain that while I could be an administrator of my space/group and have supreme control over that but only be a member of another that isn't allowed to edit pages. But possibily you could have an in house meeting for this anyway which might make it easier to explain.