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For a start you need to like the first post in the thread, but I can't see it getting much interest to be honest. I agree it would be great for vBulletin imports, maybe something could be added to the importer though.
There are significantly more permutations to handle, especially when getting into partial deletions. The importer won't be taking any more steps to merge them and I wouldn't expect to see a system to merge full conversations in XF for similar reasons.