Currently, I do not have prefix on my forum and instead use the traditional subforum. When we need an additional section for a topic, we just create a new subforum. I believe there is a point where more subforum will become too clustered and may make administration a chore.
- Permission: I can assign very specific permission for any forum/subforum per usergroup/individual. I don't know if this is possible with prefix
- Linking: We can always give the url for a subforum and get RSS feed for that subforum. It does not appear yet that it will be possible with prefix.