On a very basic level, wiki makes web publishing easier. Depending on what you are using it for, and where you install it, and who uses it, wiki can also be a powerful tool for group work- or it can be a simple way to manage the content of a prepared website.
You could use WikiAsCMS, only allowing certain people to edit, and using rigid structure and templating to control content and form. In fact, it could be a great tool, especially for small companies or not-for-profits looking for an economical solution.
It seems like a bit of a missed trick, if collaborative document editing is not allowed- but there's nothing to stop people from using WikiAsCMS.
So, uh... I think the very definition of Wiki is a content management system. It's just a particular kind of CMS that throws out most of the anal-retentive hogwash that some other ones use.
I think the thing that folks looking for an easy CMS need to think about is whether all that locked-up, authorized, authenticated bullshit is really worth their time. I mean, get over yourself, already.