...seems to be a lot like categorizing different types of music into a specific genre. 
Pete,
That is correct. When I joined the forum in early August, 2004, there were about 454 topics and 4177 posts. At the time, there were far fewer categories in the indexing system, and there were very few child boards. Nonetheless, Steve and the Moderators decided to have much greater granularity in the indexing system in anticipation that the forum might experience significant growth. Fast forward to this morning as I compose this post, there are now 18,459 topics and 195,620 posts. You can imagine what would have happened if all of those topics and posts were crammed into the original indexing system. Our members would be screaming and demanding that we break down the categories into smaller pieces.
The above said, since the indexing system was reorganized, I believe we have added only two new categories, one for Specialty-Grain Pizzas and one for Gluten Free. In both of those cases, there was substantial debate among Steve and the Moderators before adding those two categories. We have always been sensitive about not having too many categories. But, as I see it, things could be much worse. For example, Steve could have elected to have a chronological system where posts are entered chronologically. That is the system used at the PMQ Think Tank (see, for example,
http://thinktank.pmq.com/viewforum.php?f=6). That means that to find posts earlier than say, a few days, you have to scroll back through the pages in sequence or use the search engine (which happens not to be as good as our search engines). The negative of using an indexing system like ours is that someone has to look at every new post to be sure that it is in the right place. I estimate that this year alone we will be adding around 55,000 new posts.
Peter