I think going overboard with forum subcategories when setting up is part mimicry, part wishful thinking about member numbers, and part the overly taxonomical mindset of the classic webmaster.
It definitely happens a lot with these vBulletin/phpBB type sites, though I wouldn't define overboard as >1
A pretty active single-forum forum I was once part of decided to divide it up into several topics. The immediate result was a major decrease in threads and posts.