> In the file manager you can go to the "Tags" sidebar and select as many tags as you wish. So instead of a 4 level hierarchy, we would have an N combination tag set.
The nice thing about hierarchy is that it is stable as inserting a new subtree only affects very few nodes. So one can use motor and other unconscious forms of memory to quickly navigate to the desired item. A hierarchy where each node fits on the screen is perfect for a tablet as one can literally remember finger movements to get to the item. To the lesser extent this happens with mouse.
I do not see how tags provide such navigation stability. In order to keep all items on the screen (any scrolling kills the navigation speed) a hierarchical structure may need to have ad-hock subtrees. With tags those subtrees-as-tags would just pollute the initial tag set.
> Perhaps a good example is portable (and other) music players: good ones let one quickly go through the content by artist, album, genre, name, etc. as well as custom playlists. Where are they on disk? Doesn't matter.
This just demonstrates how tags nicely work with content consumption where somebody else already created all the necessary information and where there are several well-defined commonly recognizable categories to split the collection. For big collection of images the story is not so good even if one just wants to see them.