Best example for this would be IMO, a music collection. You might want to browse it by artist, genre, album, etc... Such a filesystem would make it seemless on the command line. It could as weel be achieved with symlinks though, but it would be a mess to maintain.
This filesystem might be good in some cases, but usually, when you need tags it means you have a HUGE log of data. And at this point, using the cli might be tedious (for searching for example).

I like the idea of being able to "play ~/music/tags/rock/*.mp3", but I hate the fact I'll have to run this for each song I get:


Also, even after doing that, where do I put "song.mp3" ? :)