I just moved a few threads to a hidden forum. Maybe some day I will find the time to clean them up and put them back. Unlikely.
There are two audiences for these forums:
1) the thread participants (usually about three to ten people per thread)
2) the lurkers, plus the people looking for knowledge (usually about 100 to 30,000 people per thread)
Clearly, the second audience is larger. And the first audience is more important - for without them there would be neither audience.
All of my choices on what to keep and what to delete is based on many things. One of those things is that most of the forums are building a knowledge set for the second audience (which includes me and everybody from the first audience). So chit-chat stuff that is not particularly substantial, or starts to wander off into talking about site policy or politics or whatever, or even when the topic is one thing and discussion wanders to another topic .... all of those things reduce the quality of the whole site for the second audience.
It seems like it takes all of us to put a little bit of focus on the second audience to build our knowledge set.
The meaningless drivel forum is a great place to talk about other stuff or to wander off topic.