Plone is actually a CMS.  CMS's typically don't allow the ease of editing to the community.  It is mainly used for content management on a website scale or intranet.  CMS's like plone are used for intranets, but here is where wikis are better:

-Wikis have permissions but allow easier access to edit and create data, allowing you to capture the knowledge of the employees as opposed to relying on a few to speak for the crowd

-Wikis are more dynamic than CMS with integration of other applications.  Look at Deki Wiki's, http://wiki.mindtouch.com, veru flexible API. 

-Wikis like Deki Wiki can be deployed in 15 minutes, and then you have a full blown intranet at your finger tips

-A lot of CMS's don't natively store their content in a standards format.  Deki Wiki stores content as XHTML.

If you get a chance download Deki Wiki and compare it for yourself with other CMS's.