When members use existing tags rather than creating new tags, the tagging system is less fragmented and more useful. As a result, users will create new tags rather than using existing tags. This makes the tagging system less useful and decreases the SEO benefit because of thin content.
If the right hook exists, it should be pretty simple to write an addon that would broaden the search called by autocomplete. Specifically, it would return any tag where a word in the user's string matches any substring of an existing tag.