The Anime subforum was a lot more active back in the day, and I imagine it was warranted at the time it was created (though you'd probably know better than I would). Right now you're absolutely right; If I were redesigning the sections and sub-sections from scratch, I probably wouldn't bother segmenting that either, based on current post and user traffic. Though, there is a strong case to be made there concerning spoilers, if there are anime viewers who don't read and don't want to accidentally click a thread that spoils the show for them. That aside it would, of course, be more effort than it's worth to redo all that now.
Anyway I personally don't see any convincing arguments about why what we have is insufficient or impractical, and I do honestly think it's quite detrimental to create tons of small boxes that can indirectly end up hiding content from users, or confusing them ("where do I post this?").