Although Historical societies tend to concentrate on the history of a particular area they are still valuable to genealogist. They are a must search when you are researching a particular area.
It has been my experience that the Historical Society is often the place where items are deposited when a family has determined that they have something of local interest. Due to the limited amount of funding and lack of volunteers though these things tend to spend a extended period of time in storage. They are normally cataloged, but the treasure is often times not identified. Look at it more as a treasure hunt. You might be surprised to find the genealogy treasurer that you have been seeking.