<xsl:value-of select="for ..." separator=", ">

In my XSLT 2.0 stylesheet processed by Saxon 8.3, I had this:

  <xsl:value-of
     select="for $relation in $relation-list
             return esd:xref-doc-num($mb3-connection, $relation)"
     separator=", "/>

But the results of the function calls were concatenated, without the
separator string between them.  Using string-join() like this yields the
desired results:

  <xsl:value-of
     select="string-join(for $relation in $relation-list
                         return esd:xref-doc-num($mb3-connection, $relation),
                         ', ')"/>

But why is that necessary?  Why doesn't the xsl:value-of separator
attribute work as I expect?

Thanks,
-- 
Kevin Rodgers