My take is that most sites will not benefit from a wiki.
But some may......
It's bad form to have a lot of areas of a site which are not heavily used or fully developed. Therefore, if you have a wiki....it better be good.
On my site, it's somewhere in between the form - which is a free for all, and my static articles, which are well developed. In that sense, it's a bit of a scratchpad where I and others can post things like owners manuals, the history of certain brand names (stoves and fireplace is my gig), etc.
But a forum...and eventually the RM, can serve the same purpose.
A wiki and a forum are two separate entities, and should be treated as such. A wiki is a tool for sharing knowledge, a forum is a tool for discussion. There is no benefit of just adding on a wiki and thinking it will be a huge hit. My experience suggest you need to actively promote it to your users, and be prepared to do allot of the starting grunt work.
My idea of a good integration of a wiki is to integrate it to your forum by creating a new thread for a new page, with a link, as well as posting in that thread when the page is updated. That way you have the discussion in your forum and that static content in your wiki. I have no need for a wiki, but I have previously had one running alongside another forum, and it was heavily under used, even though there was a ton of good stuff in it. I think the major flaw of it was that there were no indication on the forum of wiki activity.