In LimitRO, they use this bug tracker service and they split skill bug reports into the several classes. Over there it was just assumed that no one would bother to use the search feature and that people wouldn't bother to properly tag/label topics (and so far, they were mostly right). In the end, people actually bothered to read the reports and (excessively) repeated reports, making uncommon unless the skill was still broken after the bug fix.
Meanwhile, I go to this board's bug tracker and search for Final Strike, and I find at least three reports about the Mirror Image + FS bug and a lot of other topics where Final Strike was mentioned (even if in passing). Interestingly, I also did a normal search of another skill by simply browsing the skill board and noticed how hard it was to find what I was looking for felt to find stuff. Most things aren't even tagged to make searching easier.
So yeah, the main benefit to having the sub-categories is that you can at least reduce the amounts of repeated reports.... and that you can fully focus on one class by simply clicking the category for said class.