design question on function namespaces

I've modiified a lot of my code and moved much of the logic into 
functions. For example:
  <xsl:function name="mods:year">
    <xsl:param name="bibref" as="element(mods:mods)"/>
    <xsl:for-each select="$bibref">
      <xsl:value-of  select="substring((mods:originInfo/mods:dateIssued,
                        mods:relatedItem/mods:originInfo/mods:dateIssued,
                        mods:relatedItem/mods:part/mods:date)[1],1,4)" />
    </xsl:for-each>
  </xsl:function>

Because XSLT 2.0 and XQuery share the same data model (XPath 2.0), it's 
easy to convert between them. Here's the equivalent function in the 
eXist XML DB bibliographic example (which I use; I modified this 
function to match the logic of my XSLT one):
declare function display:year($rec as element()) as xs:string? {
       substring(($rec/mods:originInfo/mods:dateIssued,
       $rec/mods:relatedItem/mods:originInfo/mods:dateIssued,
       $rec/mods:relatedItem/mods:part/mods:date)[1],1,4)
};

I think based on this, I'm going to try where possible to have logic 
defined in functions, and in xpath (2.0) rather than pure XSLT.
One design question: Mike Kay in his book on XSLT 2.0 advocates 
function namespaces based on the namespace of the data they work on.
I ask because I'd like to keep the option open later to support formats 
in other namespaces.  So does this suggest using a namespace that 
refers to the function rather than the input namespace?
Bruce


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