sml:ref vs xlink

I All,

recently I have read SML specification and in particular sml reference mechanism in XML instance document. However I don't understand the reason of introducing a new mechanism instead of using xlink.

Are there some advantages in using sml:ref?
If yes...which ones?

That is to say...about this example:

<EnrolledCourse sml:ref="true">
    <Name>PHY101</Name>
    <Grade>A</Grade>
    <sml:uri>     http://www.university.example.org/Universities/MIT/Courses.xml
		#xmlns(u=http://www.university.example.org/ns)
		xpointer(/u:Courses/u:Course[u:Name='PHY101'])
    </sml:uri>
</EnrolledCourse>

Why not like that?

<EnrolledCourse>
    <Name>PHY101</Name>
    <Grade>A</Grade>
    <environment xmlns:xlink="http://www.w3.org/1999/xlink" 
        xlink:type="extended">
        <course xlink:type="locator" xlink:label="course"
xlink:href="http://www.university.example.org/Universities/MIT/Courses.xml
	    #xmlns(u=http://www.university.example.org/ns)		    xpointer(/u:Courses/u:Course[u:Name='PHY101'])"/>
    </enviroment>
</EnrolledCourse>



Best regards..

Travis