Do We Really Need Structured Document Formats?

Do we really need structured document formats? Structured document
formats like DITA, DocBook, and Solbook are characterized by deeply
nested tags and a multitude of schema constraints. Unstructured
tagging languages like HTML, on the other hand, are wide open. In one
meeting, every reason we came up with that made them seem necessary,
was answered by a convincing counter argument. "Reuse" would seem to
be the most important reason. And maybe there are some compelling
cases. But maybe all-out reuse isn't needed. Maybe we really only
need a very restricted form that solves those cases. In at least the
case of version-dimension reuse, variable substitution and conditional
metadata seem to be a darn good idea. And in at least the case of
table and list tags, nesting seems to be a requirement. So it's clearly
not the case that we can completely do without such capabilities. On
the other hand, the counter arguments against other forms of variable
substitution and conditional metadata remain intact -- at times, it is
just too costly to keep them working, especially in an environment that
changes frequently. And nesting everything may well be overkill, when
so few forms of nesting are actually indispensable. This post summarizes
the arguments we considered. Do they demolish the case for structured
documents in a highly fluid setting like the software industry? Do they
demolish the case for structured documents and reuse? Are they wrong
in some important respect? Or do they overlook some vitally important
point that makes structured document formats irreplacable?