We feel that descriptive metadata is often best managed in a system designed to create and manage metadata, rather than managed by mailbag tag files. This is similar to how you would prefer not to rely on embedded descriptive metadata in a TIFF file, but you would like to have some technical metadata there. Though, since a mailbag will conform to the Bagit spec, you can include any custom metadata fields in the bag-info.txt file you want! We feel that this makes the specification really useful, as these decisions can be totally left up to the professional judgement of the archivists creating mailbags.