Think of the whole internet as one vast city, like New York except bigger. Like most cities, it has the benefit of there being lots of things to do, lots of different ways to be, and lots of freedom. It also has some disadvantages. People with similar interests are scattered all over the web (city), and it's not always easy to find one another. Even when you know where the people you want to be with are, or the sites (stores, places) you want to go to are, they're all on different sites (in different neighborhoods), and it's cumbersome to link (travel) to them. Their addresses can suddenly change, and you won't know where they've gone. Also, the costs (rent) are often high, since everyone has their own site (apartment).
A well-maintained wiki based on a particular theme or interest can act like a village for the interest. Linking is much easier. Since resources are being shared, costs are lower. Starting a page (house) on a wiki is much easier than starting an independent site. And the wiki is human-scale, unlike the city. Of course, there will always be links out of the wiki to the wider web, just like people can go into the city if there's something there that the village doesn't have. But you always have the village to come back to when the city becomes too much.
This model leads to an idea that probably wouldn't have been suggested by the "wikis as the internet with write privileges" model: wikis as a place for quick and easy hosting. If you have a group (in CyberSpace or MeatSpace) that could use a webpage, but don't need anything very fancy, you should be able (I think) to just start an official home page on the canonical wiki related to what the group is about. You have instant connection to the community -- you can put links to and from your page on the relevant pages. Other people can link to you very easily. You don't have to pay 10 bucks a year for the domain, or anything for bandwidth. And if necessary, on some wikis the page could be protected, or only yourself given edit permissions, so that it can function as an official home page without fear of some joker changing dates or contact information on it.
I starting thing about this model as I was thinking about the vegan/animal activism/vegetarian wiki I started, WikiVeg. I didn't just want to have an expanded vegan Wikipedia, but something that would function as a sort of hub for the entire veg-related internet. Rather than having to travel through the big, lonely internet from site to site for all the different things related to veg*n living -- one site for recipes, another site for statistics on animal abuse, another for activism resources, another for meeting people, another for restaurant reviews, etc. -- a veg*n should eventually, I hope, be able to go to Wikiveg and find everything s/he could want in once place, with outgoing links in case there's things other sites are doing better. And anyone who starts a Smalltown Vegetarian Society should be able to instantly put an official homepage at wikiveg.org/Hosting:SVS if they like. Right now Google is a pretty bleak portal to the vegan internet (or to anything really) and the existing veg sites are only a little better. I want Wikiveg (and other wikis I have planned) to be the best all-in-one site and the best portal to other relevant sites. --ZachAlexander