One of the proposed WikiPolicies.
Zope 2 wiki is more useful as a body of clear documentation than a discussion archive. So refactoring and deleting comments is often the right thing to do. This means extracting the useful meat from the comments, integrating it within the main body of the page, and deleting the old comments. When to do this is a judgement call. BeBold. Do refactor the comments of others.
Comment with care. Every comment you add provides some (variable - hopefully positive!) value to the wiki, especially in the short term. But, it also incurs a long-term debt for clarity and maintainability. Don't keep adding comments without occasionally doing some integration, especially of your own.
Pages ending in "...Discussion" are exempt from this. In general, if you hope to preserve your comment for the ages, it should be on a Discussion page such as ZopeWikiDiscussion2005.