There has been a lot of discussion about Wikipedia of late. Much of this has been about 'authority'. There is another major issue at play here as well, which is really quite interesting for libraries.
Let me step back a moment to talk about the URL. The URL is the currency of the web. For something to be referenced, to be talked about, to be shared in the web environment, it needs to have a URL. Those things that are referencable are more likely to enter the web conversation; those that are not referencable in this way are off-web and much less visible.
Wikipedia is an addressable knowledge-base. It allows me to incorporate additional 'knowledge' in my communications by simply including a URL. The economy and convenience of doing this is enormous, and it is only possible because the resource is on-web.