Au contraire Pierre. Those are exactly the kind of users you want for "long term growth," because they care about the topic and want to participate.
One of the forums I run has all the topics related to the subject open for public view, but the small handful off topic forums are register-to-read. So Google crawls all the relevant forums, but the "community" type conversations are for the community.
Which records the forum users are listening to, or which movies or TV shows they watch has nothing to do with the subject of the forum so it doesn't have to be indexed by Google. The users like it that way, because they can talk about things they don't necessarily want to show up on Google.