What we haven't done yet, though I'd like to eventually get there, is to be able to take files that have been organized via filesystem hierarchy and ensure that gets reflected in the semantic data. We've story boarded some of this already while designing these parts of the UI, so it should be possible.
However, most use cases on tablets don't have that use pattern at all. More common is slapping in the SD card from a camera and viewing pictures, perhaps organizing them as you go. Downloading files (e.g. books) from the network is also common and this also leaves you without structure by default. These are the common use cases on mobile device which "organize it yourself in the file system" as an idea really sucks for. Semantic organization turns out to be far more powerful there.
But that's the use case for devices. In Plasma Desktop we have these same features (it's the same code under the hood, after all) but we still present a file system centered view by default because the Desktop use scenarios are different from mobile, and often follow the kind of thing you noted.
In future, I really do want to be able to go in both directions, however: sort your pictures by tags and then be able to sync to, say, owncloud with pictures sorted into folders by tag sets so that you can load them on any desktop / laptop PC with your structure intact; or go from a file system sorted file set and be able to identify those as tags that co-exist with metadata.
Anyways, key here is to keep in mind the use case of the target devices. They differ because of how we (humans) work in different contexts, and we tend to choose different device form factors for different contexts. (Tablets being frustrating to use at a desk at work, while it's inconvenient to drag your laptop with you to the park just so you can see a map of how to get there.)