One case where I think it is useful to start a new thread, even though the answer you seek may be buried in other threads, is when you want song lyrics or some other piece of information (as opposed to lenghty discussions), and your forum search turns up 17 threads, none of which have a title relating to your question, and each one having 100 posts.
It may take you a long time to wade through all those threads to find the lyrics or answer you seek, when if you post a new thread, someone will probably answer you within minutes (often giving clickies for all those other links, as well!).
Genie