An extension makes more sense to me than a core feature. Let's take a support type forum like phpBB.com forums. You'll end up with many related topics since both the type of questions and the post content (due to the support template) are very similar. You'll get many "false positives" for topics that seem related but in reality are not.
Since it will increase server load, many of these forums probably don't want such kind of feature. Therefore: an extension is the logical option.
IMHO, this goes for most of extra features of this sort: they can be very useful for many forums, but don't really add anything to the base product. It's an addition to the base product, and those should remain as extensions rather than a core feature. That will keep the system fast and agile.
Above message may contain errors in grammar, spelling or wrongly chosen words. This is because English isn't my mother tongue. My apologies in advance.