Thank you for the answers that
confirmed my understanding.



I didn't mean to omit XSD (it's actually the one I know more), but with
reference to technical documentation, the main two current 
applications use DTDs (DITA, DocBook to 4.5) and NG/Schematron (DocBook 5.0).



I guess for authoring purposes, in most cases industry standard schemas
will suffice. Especially with DITA specialization feature. Probably,
not in too many instances that a business will
choose to develop their own schema (generically speaking, incl. DTD). 



From a schema development perspective within this scope, it seems to me
that it is more of concern to broad info architectures and tool
developers, and may be content management. But one motive I see for
developing own in narratives or paginated documents, is that
customization and/or parameterization of standard ones isn't very
straight forward. Developing own schema that is more requirement
specific simplifies the development of scratch transformations to
assume better control over publishing appearance -- unless investment
in high-end tools.



This is really the niche where I was interested in finding more about
how likely/often a practitioner would need develop custom schema.



Any further comments appreciated.



Regards.

Khaled