Ward Cunnigham in an article on perl.com explains why he invented the concept of a wiki:

Back in 1994, the Web was a pretty wonderful place, with lots of people putting up stuff just because they thought someone else would find it interesting or useful. Wiki preserves that feeling in a place that has become too much of a shopping mall

Amen to that. The graphics/content ratio of the web is too high as is the commercial/noncommercial ratio.

Wiki's are being used for quite wonderful things - from the hard to navigate original pattern repository - with material of very variable quality, to the surprisingly serious Wikipedia a free (as in freedom) online encyclopedia wikied together by voluntary submissions (no less than 130094 at present in the english edition - many partial translations are also being worked on). In fields where geeks are good resources the material is of very high quality - often surpassing the material in some of the other encyclopedias I consult on a regular basis. In fields where an editorial stance is required, e.g. politics, it is less so. But even that is an interesting departure from standard encyclopedias - clearly in an open editable medium you will have competing factions fighting over the meaning of politically charged terms.
Wikipedia has augmented the wikiprocess with a dispute resulution process to settle such issues. Pages in dispute carefully display a notice that they are.