The 'version history' or 'revision history' wiki feature ('view other revisions' link) displays changes made to a particular page, and allows any user to retrieve an older version of the page.
This feature is essential and is one of the defining features which allows the wiki concept to work at all. EditSummaries are also a basic feature which should really be incorporated into the version history display.
Some really primitive wiki engines don't have version histories. These should not really be taken seriously. In fact they give the wiki concept a bad name, and should never be installed as publicly editable wikis, since there is no protection against spammers or other vandals permanently destroying legitimate content.
It may be that old revisions are kept in files available to the server administrator, but this seems pretty shabby, and from a spam-fighting point of view this is completely inadequate. It's too much work for the administrator, if the administrator bothers at all. If the administrator is at all slow in cleaning up spam and reverting vandalism, then this will quickly become frustrating for any other users.
Wikis which have this problem? We probably shouldn't credit them with a link.