This seems a very good and important point.
If the problem is how to represent occassional structures in well-formed
documents, then AFs represent an external form, XML-data represents an
inline form, and ISO 8879 declarations represent a header form.
But, I think that a document with AFs cannot be regarded as being
declaration-less, since either the declarations have to be implicitly
built into the application, or be explicit in the form of a DTD outside.
The horrible thing is that, of course, there is no reason why an
XML-data schema could not itself be a meta-DTD! I think the
issue of direct modelling (SGML templates or XML-data) versus
indirect modelling (AFs) should be distinguished from the issue
of the goodness of ISO 8879 declaration syntax versus XML-data
non-standard syntax. AFs, as a mechanism, are syntax-neutral
to a great extent.
Rick Jelliffe