An example where this would be useful: in writing short intros on some pages in NeuroWiki, sometimes I write things that the target audience of NeuroWiki (people in or entering into neurosciences research) may find too introductory. I would expect that if NeuroWiki ever gets off the ground, those pages would become something else entirely from the short intros that I put up (i.e. more a page about current research on the topic than an explanation of it). Wouldn't it be nice in that case for the odd neuro-beginner to be able to read only the explanatory part?
As I always seem to preach, I always thought the HyperCard system of beginning and advanced modes was a good way to have powerful software while still making it approachable. ViewPoint with different "depths" of expertise as different views would take this approach to hypermedia as well. I suppose this is part of the MakeWikiMoreAccessible discussion, although it is about making the content more accessible, not the Wiki idea itself.