At a conceptual level, a tree-structure of tags is just meta-tagging. You apply the "Family" tag to the "Kids" tag and the "Home" tag to the "Family" tag, and

At a conceptual level, a tree-structure of tags is just meta-tagging. You apply the "Family" tag to the "Kids" tag and the "Home" tag to the "Family" tag, and you now get that much more information out of applying the "Kids" tag and can filter more efficiently. (E.g. filter all "Home" tags out of a view, rather than itemizing each and every tag that you imagine falls under the idea of "Home stuff") The tree is then just a visualization of relationships and not a strict hierarchy. I don't know if Google has implemented them this way (I'd like to hope so). But if they have not, it's not a failing of the idea.