Just FYI the metadata that gets written to the XMP files do not include the full history of edits that were made on the photo.I was always under the impression that XMP was sort of a backup of all my edits, flags, etc. that would stay with the image no matter where it went, but it turns out this is not the case. It is true that your edits get stored – whatever the most recent state of the image is in Lightroom, including snapshots. – but the edit history does not get recorded. Neither do your flags, ratings, or collections info.
I still write changes to XMP, but now I know I can’t rely on that as an alternative to a full backup (or multiple copies of a full backup!) of my actual catalog.