Hmm.. True.. My first thought was well, i could say the reverse, but realized, our current implementation of directories and hierarchies are restrictive. in the sense of having multiple tags/directory names to a set of files(it can be argued, symlinks do the job well enough). The analogy, also brought to mind the choice to allow multiple inheritance or not. I guess the trade-offs are the same. Power to the user vs easier(but restricted ) for the user -- I don't mean the tag system per se, but the idea of having only one level of filesystem.

P.S: As far as my personal stance, it comes down to the type of use/nature of application/(perhaps more appropriately) amount of attention being spent. I don't want to have to descend hierarchies of filesystem(Even ones, i created/customized myself) to find my music. OTOH, I'll be pissed, if i have to search for my code files by tags/file contents.