The Problem I see with this Nested Notebooks vs Tags is that, when a team is working together, a teammate has to either follow the nested folders and find the logical place to go or try to decide which is the most logical tag to follow the laid out path. Nested Notebooks leave no room for interpretation while tags can be ambiguous. I can see clearly that the tags year, Invoices, Dept, Vendor is the most logical way to follow, and someone else might think Invoices,Vendor,Year just make sense. With nested books. One path. Anyone can follow it and understand it. No need for additional communication in the form of explanation of the way things are organized. Just follow the nested books and make sense of it.