I think this is exactly right, and it is a facet of the same discoverability issues that crop up when people talk about GUI vs CLI - one is more useful when you're discovering, and one when you are searching. Tags are really set-based search operations like a SQL query, but the 'primary key' is the filename, and if you knew that you'd just search for it. You're rarely going to have a tag or attribute that can pinpoint a single document.