We need cleanup and merging of questions more than ever, and I suspect we'll need it even more as the site continues to grow. Take a look at this noob-ish set of near identical questions:
I could go in and manually add a comment to each that listed all the other similar ones. That would be a lot of work on my part, and thus not maintainable. Further, it's not a great UI for users finding the information, and it doesn't give the site the metadata of relatedness.
My first proposal is that we add for 10k+ or 20k+ (or perhaps higher) users that allows them to perform a search and use checkboxes to flag a set of highly-related questions.
Separate proposals would be ways of how to use this information on the site, and how to display this information to end users.