+1
Some broader thoughts on the matter:
More forums inevitably thins thread distribution out. If you're unimpressed by 7 threads having replies in a given forum on a given day, that number will only go down if several of those threads find their way to a genre-specific sub-forums.
The general rule with forum creation is that it's employed to deal with topic overflow. If there were tons of Horror-related threads in the General Movie Discussion forum, to the point where people uninterested in horror had to comb through them to get what they wanted, then it would make sense for a Horror Forum. But as it stands now the General Movie Discussion forum has a good cross-section of topics and isn't flooded with any one thing; which is the goal with any "general" forum.
Another thing to consider is the possibility of fracturing the user base. It may hard to be imagined now, with this fairly eclectic group, but the clique mentality was a huge issue a few years back. Many new users felt they were shut out by the regulars, and that they had to post hundreds of times and visit regularly for months to feel "accepted" by the community. More forums and more topics, while they would have some benefits, carry with them this possibility, albeit in smaller doses.
Anyway, I don't want to give off the impression that I'm dismissing this out of hand. I'm not even ruling the possibility out. But my first instinct is that MoFo has struck upon a pretty good balance of forums and hasn't had topic overflow problems for quite awhile as a result, which is no small feat.
That said, there is one big exception to all this: section moderators. Mods around here help cleanup spam and the like, but on many forums it's very different. On most forums (especially very large ones), moderators essentially run a specific forum or category, and are sort of in charge not only of keeping it clean, but also keeping it active. If members here revealed that they wish to run a genre sub-forum, and have been actively holding back threads and posts because they didn't feel they had a place here, that'd be another story. I'm certainly open to sub-forums in that scenario. If this is the case, let me know.
In conclusion, this is a very tricky business. There's an ebb and flow to communities like this and it's very hard to pick up on. I don't want to play the experience card too blatantly, but I've been doing this for a very long time and have taken on every role in the forum hierarchy on various sites, and even I wouldn't pretend to know how it all works. So while it may sound "ridiculous" that activity would be reduced by this idea, JB, it's honestly not absurd. I have my theories about why this might be, but community building is more art than science.