@codinghorror yes, i have read the various discussions on the site about the relative merits of categories vs tagging and agree with the direction that you have been pushing. SO currently has about 1200 tags which means practically search is the only way to navigate. the discourse approach of categorisation plus tags is much more efficient. also, i agree that too many sub categories can hinder rather than aid navigation.
but my question relates to allowing a topic to not be restricted to a single category for situations where, as in the example, the topic could logically reside in more than one category. tagging is a workaround but doesn’t fully substitute as tags require search, which is what the categories help avoid (and categories are a more subtle way of presenting users with relevant topics compared to having to actively search a tag). i believe a one topic to multiple categories capability would improve the UX without compromising the logic that led you to adopt categories in the first place.