Now Movable Type supports tagging features natively. But, existing XMLRPC/AtomPP-based blogging tools such as ecto and MarsEdit cannot employ tagging, because they don't recognize how to add tags to entries via XMLRPC or AtomPP at this moment.
To make it possible to posting entries with tags, KeywordsAsTags Plugin provides the feature to convert 'entries with keywords' to 'entries with tags' automatically. So it can work as an adapter for tag-disabled blogging tools!
Movable Type versions: 4.3