I don't think things should be deleted. It lets people see why, when etc. things got discussed and how decisions got made. If you have a query about something you can do searches and see whether it has been discussed before. Deleting discussions somewhat defeats the point of having a discussion board doesn't it? Juts my thoughts. If you were to delete things for tidying purposes it should be minimal and be things like discussions on planning events that happenened 2 years ago or something.
those are me thoughts.
It's like God's vagina!