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.
 
  Eric