Need to import the XML schema namespace

Most XML Schemas in the wild reference some of the types in the
http://www.w3.org/2001/XMLSchema namespace, but they usually do so
without explicitly importing it, so it apparently unnecessary in
practice, if not normatively.

I can't find any reference in the XML Schema 1.0 spec that says it is
either illegal or unnecessary to import this namespace, for example,
using xs:import.  Similarly, I can't find any further interoperability
guidelines on this subject in the WSI-Basic Profile 1.0.

Does anyone have a reference to a definitive statement of whether this
import is required, optional or forbidden in XML Schemas that reference
the data types of this namespace?

Jeff