datatypes and values without lexical form

The XML Schema WG have asked that we require every value have a lexical form.

http://www.w3.org/XML/Group/2003/03/xml-schema-rdf-notes.html#ab2b3b3c11b3

I am trying to remember why we don't.

IIRC it was XML Schema Union types that were the problem.

Did we have other reasons?

Jeremy