Perhaps but take the idea further....
Say you have tags of
Comedy
Comedy, Romantic
Comedy, something else
Comedy, another something
Which is easier ?
The positive way is to go directly to Comedy, you see a helpful twisty indicating there are subitems, drill down and go straight to what you want. not to mention the close proxmity of all subtypes, in one place.
What isn't clear with the subtractive way is you dont know how many different types of comedies there are to begin with, assuming some one else was using the system.
So you click on comedy, then get a huge list and wonder what exacly you are seeing. There is no indicator that there maybe different subtypes of comedy in that list. Its just all comedy. How far away is Romantic from Comedy, depends how many other genres lie in between
Thats why i think the positive approach is clear and therefore preferable.
I have no objection to a subtractive way of doing things just that i can't immediately see the utility of it in the example given above. It works for maybe two or 3 types of comedies but if you had 10 types then the only utility i see is taking away one or two from many, So going to the first post, removing Trance from the list.
The main use being in conjuntion with (All) and subtracting say one or two items from it.