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.