Seems to me that what is needed is consistency. A hierarchical file systems represents something metaphorically 'physical'. A tagging system seems to me to be t

Seems to me that what is needed is consistency. A hierarchical file systems represents something metaphorically 'physical'. A tagging system seems to me to be too ad-hoc, and therefore untrustworthy. If I know I left a file in a folder, I know it will always be there. If I tagged a file with some sort of mnemonic, then it's on me to remember what burst of inspiration told me to tag it as such.