There's a thread somewhere around that talked about the tagging system, but taking that further and having a list of similar threads based on tags. I think that would be really useful rather than just a basic similar threads section.
I think a similar threads feature would be useful. Also maybe have a feature on the new thread page that checks to see if there are similar threads already and alert the user before they post in order to cut down on duplicate threads.
I've recently turned it off on a lot of sites. The statistics shown that 95% was search engines / crawlers, and from the 5% it were incidental clicks. Clutter on the page for me then, so turned it off. People rather use search (so it seems) to find the content, can't find it? They (hopefully) post about it.