I believe what is needed is an explanation how this or other feature would be usefull. Let's take SoPW categorization:
- when you ask a question you don't really want to narrow the audience
- when you answer you might want to narrow the questions to those that are in the scope of your knowlege, but actually I like to read all the questions and then browse the answers - to learn something new
- only when you are searching the database for discussions it would be really usefull