Well, I'm not sure what your model of a document is, but mine focuses
more on information, and how it is both represented, and presented.
I would say that there are two major classes of documents: those for
which appearance is of the utmost concern, and those in which
information is of the utmost concern. In the latter category,
technical manuals, novels, journals, etc. all fall in. Most of
the data in these cases is long-lived, and fairly static. In such
cases structural markup (read SGML) has many benefits.
Admittedly, this is not the major focus of the WWW *now*, but the
focus will change over time: not only change to support SGML, but
change to support other visually oriented formats as well.