ayg wrote:

Why not just use [[Category:Physics 101 exercises]] or whatever? We definitely
don't need a new syntax for this.

Bernhard.Fastenrath wrote:

Admittedly it is just syntactic sugar. I had several collisions already (not in
wikiversity) where I would have liked to use the same name for different
categories and at the same time didn't want to make category names arbitrarily long.

As an example:
http://education.wikia.com/wiki/Category:Learning_environment_computer_science
would initially have been a category "computer science" only visible to
educational software. In the same wiki there is a need for a category "computer
science" for school subjects. For educational software I use a lot of categories
(a tendency which will probably increase even further) so keeping the category
names shorter becomes important and I don't want to use abbreviations
(Category:LECS ? - so what?)

Without generated intersections for categories (Bug #5244) I have to give longer
names to categories that seem inevitable for navigation.
There are other applications where I could imagine that this would be quite
useful (e.g. wikiversity)

robchur wrote:

On the whole, the better solution to this problem would be to allow intersecting queries, as bug 5244 requests, thus I'm closing this in favour of that.

I don't think it's a great idea to add complexity to an already overloaded syntax, and I'm doubtful that the work this requires has a great benefit for the software overall.