The ability to disable Browse and Search in a given space, will allow creating wiki-islands incubator-type spaces. Such 'island' pages (isolated wiki pages with no visibility into the space beyond/no birds-eye view on the wiki) can be shared within a team/community/group of users (by sharing the URL) without having to share the whole space with everyone at once/the same time (unlike personal space pages). Rely on word-of-mouth interlinking to other pages in the space, for making pages accessible from each other only within some kind of context (vs. cold search results).

Benefits:

• Allows topics to emerge on one or more related pages ('islands') before moving out of the incubator into a relevant "traditional" (and searchable) team space.
• Encourages interlinking between pages which are related (results in better inter-page browseability and provides more metadata for search relevance and "related pages" functionality - once the content moves to a searchable space).
• Encourages using red (undefined) links to first map out future related/children pages (i.e. top-down mental process).
• Allows unrelated pages to remain invisible to each other within the same space, in turn allowing to share a page (and those it links to) without necessarily sharing the whole space. This can also reduce the overall number of spaces required.

Of course Space admins would still have god-like browse and search access to everything, and 'Browse & Search' would be ON by default to keep current standard behavior.

I know it sounds contrary to wiki philosophy to wish to restrict page browsing (i.e. content discovery) but as mentioned above, it can make sense in some cases.

Note: Comparable functionality is available in Picasa Web albums: one can elect to make a photo gallery public (visible on the user's public gallery index page) or private (gallery still publicly accessible without logging in, but URL obfuscated so it's hard to guess and virtually impossible to randomly discover). The key is obscurity (content hard to find unless you've received a link to it), not restriction (i.e. read/write permissioning unaffected).