I think wikis a great for collaborative writing. The main plus they have over most CMS is :
- revisionning / rollback
- auto TOC generation
I also agree comments on the documentation would be nice, but more in the way some wikis have with "Discussion page". I think it’s better not to clutter the documentation pages themselves.
If we need/want a wiki used with MODx, we can try integrate one, but since MODx will at some point have revisionning/rollback, would it be of interrest ? Not sure... plus the already discussed issue of wiki syntax : some people don’t like them at all... we had the problem with TextBook (Textpattern’s wiki). Contributors, used to Textile, were put off by MediaWiki’s syntax... It does not mean it can not work (see WordPress Codex, which is great), just that context is of the essence when choosing a web application.
But true it’s something that’s very much worth discussing, which indeed has been discussed among the MODx team
Can’t vote Yes or No, just say that maybe in MODx’s case it’s not worth having a wiki for we can do it an alternate way.
About the web 2.0 angle, blog and wikis are "trendy", as is AJAX but different tools have different use (it’s deep, isn’t it
?). Forums will remain useful for communities and won’t be replaced by wikis or blog. Though it’s true web applications tend to go toward simplifying : in an over-informed world, signal vs noise demands more simplicity.
Vanilla’s succes is a nice example of this trend...
This being said, if you are interrested in web 2.0 + wikis => check
StikiPad out