Fight nice, children.
Rick, your solution is nice, and clever. And I've thought about implementing essentially the same thing often. Whenever I'm frustrated by the existing system (which just happened to be this weekend, and then here was this thread).
The main problem for me with that system is not in displaying or using the metadata simply to construct views (which your solution solves beautifully). It is that it would require me to use a certain tag for some kinds files, and another tag for other kinds of files, when
applying the tags and creating the metadata structure.
The task of tagging is something I do far more often than designing views, and with far less enthusiasm, if you know what I mean. I prefer to build metadata systems that are both simple to use and efficient to
apply. That generally, makes them also simple to
understand.
To use that, I'd have to have special views for tagging these [Media Sub Type]=Blank files, and separate schemes to tag the [Media Custom Type] ones (or whatever suggestions you gave for the custom tag names). I can't do it all in one pane, in one view. If I'm using the Columns or Tag AW to add tags, I have to show both (or again, make special "column setups" for each special type). If I'm using Pane tagging, then I need either an extra column (which are at a high premium in big tagging-centric views), or a special view custom designed for that "type of file".
So, to be clear,
you could make this work, if you were clever and determined (you are, and frankly, so am I when I put my mind to it). But one of my rules for setting up my organization scheme is this: Can I explain it to my wife
once or twice and have her be able to understand the system well enough to use it
and tag files herself properly. I try not to design any system that requires
me, above all else. Because if it requires you, then it is
only your job. And that job is often annoying. Even if she doesn't do it often, and messes it up a little when she does, every little bit helps (and it keeps her involved which helps with future WAF). Plus, sometimes I tag files when I've had too much wine.*
So, I try to stick to the assigned Media Sub Types as much as is possible. They're required, so I have to use them. I don't want to use something else as an adjunct, because then I have two separate tags to worry about when applying tags.
* See what I did there, Rick.