Well, I'm working from the premise of a gaming wiki, so I'll elaborate with that in mind.
I have a portal page for a game system, which is intended to be the gateway for all material that uses that system. I want to create a collection of links to the content on my wiki, broken down by category, but I no longer use categories like "<game system> characters" or "<game system> campaigns," which have been made obsolete by SMW and SD. I now use the semantic tag "uses game system::" in each article, and the articles are categorized into a much simpler tree (Category:Characters, Category:Campaigns, etc.). On Special:BrowseData (hereinafter, "S:BD"), these categories are filtered by, among other things, game system.
Now, this led to my other request about direct links to S:BD with filters already selected. I want to link to lists of articles that all have "uses game system::" set to the system on my portal page. I didn't want to create the external-style links (though I may yet switch to that), so for the present I'm using #ask: queries to search for articles that have the right property value.
But it occurred to me that it would be really nice to be able to further filter the results. If I search for a list of characters under the game system, perhaps I'd like to filter the results by which campaign they appear in, for instance, or where they reside. I can arrange the search results into a table to show that information, I know, but that's not a real filter. It is also inconsistent with S:BD being the primary article browser around which my wiki is organized. I'd like to keep a consistent interface for my users as much as possible. They're not very wiki-savvy in the first place, and keeping things as simple as possible would be better.
Anyway, direct links to S:BD with filters already selected would probably take care of me in almost every case. Now that I dwell on it, the only situation I can think of where a Drilldown Search would be really handy is if I searched for articles that fell into more than one top-level category. For instance, if I search for every article that has "uses game system::" set to the right value, I'll hit articles in about seven top-level categories. Then I could browse among only those articles on S:BD, even switching between top-level categories, without seeing content for other systems or no system at all.
For what it's worth. It would probably require more work than it would return in functionality. :p