In general I think the subdivision of a forum into (too many) categories is not conducive to a particularly healthy community. Some categorisation is good - people seem to need a bit of structure - but the line between the right amount and too much can be quite a fine one. Particularly as somewhere gets bigger and more people want more categories, it's all too easy to create silos where people don't stray outside to see what is in the other side of the fence. The other side of the fence is, to me, where often the most interesting conversations and learning take place.
Having said that the forum software used here does seem to work quite well in stopping this. The general view of all categories in one list, opposed to the more segmented view of a typical phpbb forum for example, means that (unless you specifically mute a category) you are more likely to see/read/respond to things whatever category they are in. Certainly this is true for me anyway.
No disrespect to @kisielk's idea but I'm honestly not sure what benefit it would bring. The "no-category" category is full of fine conversation on interesting topics that could quite easily be a place for discussion about composition or creative music practice. I don't think having a category for these is going to elicit a different response to such topics from members here, although I'm pretty new and could of course be wrong!
I could go on but I'd be rambling even more than I have already