I don't see why we need to preserve those that have nothing but the request and a link to the DT or another thread, although what happens if the person who originally posted comes back looking for their thread?
I would also be reluctant to see large quantities of threads edited and combined, even though it may seem efficacious. So many times a thread will go off on its own to incredible realms of knowledge, sometimes related to the original posting, sometimes not...I'd hate to see any of those be combined and sort of lost in the shuffle.