LISTSERV produces digests in the standard RFC1153 digest format, which is
convenient for all users rather than just those with MIME. I realize that
a MIME digest would be more convenient for MIME users, but it would also
be a lot less convenient for the vast majority of non-MIME users.
Furthermore MIME user interfaces are generally pretty sophisticated and
it should be easier for MIME users to tell their user interface to
automatically move messages from list X to folder X so that a normal
subscription would achieve the same thing.