I never lock threads. Often, duplicate threads get even more useful replies than the original because people don't have to get up-to-speed on a huge topic. "Completed" topics might benefit from new information.
Off-topic posts are a problem, but I'm not sure how to deal with them. I don't want to delete them on sight, since that causes confusion and makes people upset. The split/merge tools are not flexible enough to deal with larger cases without ruining the discussion.
This makes a lot of sense. What about when we have an influx of noobies and 3 threads about the horrors of lost coins appear at the same time? Are there some occasions for lock/combine?
I think I'm going to make an list of threads about the most common topics so I can quickly link new threads to the relevant thread. If I get much done I'll share it. Another way to do it might be to pull out good answers from the thread, put them in the wiki and link there to avoid linking newcomers to long ass threads.