Question about Import & Variable Schema Definition

Import is currently defined as:

<complexType name="tImport">
    <complexContent>
       <attribute name="namespace" type="anyURI"/>
       <attribute name="location" type="anyURI"/>
       <attribute name="importType" type="bpws:importedURI"/>
    </complexContent>
</complexType>

Shouldn't it be?

<complexType name="tImport">
    <complexContent>
       <extension base="bpws:tExtensibleElements">
          <attribute name="namespace" type="anyURI"/>
          <attribute name="location" type="anyURI"/>
          <attribute name="importType" type="bpws:importedURI"/>
       </extension>
    </complexContent>
</complexType>

Also the schema currently says:

<complexType name="tVariable">
<!-- variable does not allow extensibility elements
because otherwise its content model would be non-deterministic -->
	<attribute name="name" type="NCName" use="required"/>
	<attribute name="messageType" type="QName" use = "optional"/>
	<attribute name="type" type="QName" use = "optional"/>
	<attribute name="element" type="QName" use = "optional"/>
	<anyAttribute namespace="##other" processContents="lax"/>
</complexType>

I looked at the schema and I must admit that I don't understand why 
making tVariable extensible is a threat to the model's determinism. What 
am I missing?

	Thanks,

		Yaron