Wikis are a great tool for sharing information and having it in a central location - the director of my library and myself put up a wiki (we used wikimedia).
In my opinion, schools should not be banning wikis as sources of information - but rather teaching students how to be responsible consumers of information. This is one of my top ten pet peeves. (That is if I was organized enough to have a list!) Students should learn to be discerning in how they rate their information resources - the format of anything, book, newspaper, wiki, should not make it credible or not credible. Just because it's a book with a hard cover doesn't mean the author knows anything --- and just because any joe can publish on a wiki doesn't mean he isn't knowledgeable about his material.
If we are taught only certain resources are "good" aren't we just getting lazy and becoming enabled to not think for ourselves? Ack! *hair pulling*