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With this function I could have 'parent' tags of 'location' 'genre' 'Star Trek' etc. and then have 'children' tags of 'box 1' 'box 2' 'science fiction' 'fantasy' 'ST:TOS' 'ST:TNG' and so on.
For example, Articles of the Federation isn't based on any one Trek TV show. If tags were hierarchical, I could just tag it "Star Trek", and have "ST:TOS", "ST:TNG", etc. as children tags of that. With a "Star Trek" tag group, I'd have to have an "other Star Trek" tag for books like AOTF or the Star Trek Encyclopedia, which sounds rather ugly-looking to me.
Hierarchical tags would also let me do some fun stuff with the Star Wars tags, letting stuff like the Clone Wars novels automatically inheriting the tag for their era.