What's to prevent someone from going berserk and wiping the whole site, or secretly changing the meaning of what people say, or clogging everything up with spam?
The reason wiki works is because community members engage in PeerReview. Most wikis store old versions of each page for at least a short period of time, allowing damage to be easily recovered (see KeptPages). If someone comes along and deletes text or posts spam, someone else can just as easily fix the problem.
Wikis work because they rely on the community, rather than technology, to police itself.
See WhyWikiDoesntNotWork for more details.