Writing metadata to file simply embeds the keywords, captions etc. The data then travels with the file if you use it on-line or in another application this can read metadata. That does not involve the trash so you must have made a mistake.
Last updated January 21, 2011 Copyright 2008-10 by John R. Ellis
In my opinion, the real risk is to forget that "writing metadata to files", which is possible in most editing softwares, is NOT enough to keep your organization; this writes the keywords, captions, notes, ratings to the files, but your albums, stacks, version sets and creations are not and can't be written to files. They are kept in a catalog and the only way to move your media files folders as well as your catalog requires using the backup and restore function of the organizer. Never re-import in a new computer even after writing metadata to files, use backup and restore.
That said, writing metadata to files may be an additional safety process and it's the general way to share files with tags or captions between different softwares.