Well, I guess that optics is covered by classical and quantum physics forums concerning the basic physics, unless it is so material-specific that it belongs in condensed-matter physics. It can also be engineering related, or application-related (general astronomy). Usually the optics problem at hand defines quite well in what area the post belongs, no ?
We try to set up the sub-forums in such a way as to organise the incoming flux of posts in the best possible way, with as low a number of forums as possible. If there were indeed a huge flux of optics related posts or if there were serious difficulties in hosting them in one of the existing forums, that would justify creating a forum for them, but given that this is not the case today, we don't want to multiply the number of subforums. It complicates maintenance and readability. The day that the necessity shows up, we can create it.
In my opinion, most discussions have to do with different principles behind a cross-forum subject such as optics. These principles are usually confined to one of the forums in the physics or engineering category.
I agree with vanesch that having as low number of forums as possible is a good approach.