Bruce,
 
The ID attribute on the mods element should probably not be used for any "descriptive" identifier.  That information really belongs in the identifier or recordIdentifier elements.  The ID attributes sprinkled throughout the schema, which are included on the elements mods, titleInfo, name, subject, and relatedItem, allow implementors to link to specific parts of the record by using the XML ID/IDREFS data types.  The use of mods in the context of a METS document drove the inclusion of these attributes.  Below is an abbreviated METS document to illustrate the linking from the structmap section of the METS document into the mods record.  The bolded attribute values demonstrate the links. 
 
Corey
 
<mets:mets PROFILE="lc:score" OBJID="loc.natlib.ihas.100010802">
  <mets:dmdSec ID="DMD">
    <mets:mdWrap OTHERMDTYPE="MODS" MDTYPE="OTHER" ID="MODS1">
      <mets:xmlData>
        <mods:mods ID="MODS1">
          <mods:titleInfo>
            <mods:title>Library of Congress march</mods:title>
          </mods:titleInfo>
          <mods:name>
            <mods:namePart>Sousa, John Philip</mods:namePart>
            <mods:role>
              <mods:text>Creator</mods:text>
            </mods:role>
          </mods:name>
          <mods:originInfo>
            <mods:dateOther>2003</mods:dateOther>
            <mods:issuance>monographic</mods:issuance>
          </mods:originInfo>
          <mods:physicalDescription>
            <mods:form authority="marcform">print</mods:form>
            <mods:extent>1 score (12 p.) + 23 parts ; 8 1/2" x 11"</mods:extent>
          </mods:physicalDescription>
          <mods:targetAudience authority="marctarget">adult</mods:targetAudience>
          <mods:note type="Copyright_note">Copyright 2003 by John Philip Sousa, Inc.</mods:note>
          <mods:note>Printed score and band parts.</mods:note>
          <mods:note>The march was begun by John Philip Sousa in 1931, but never completed.  Stephen Bulla was able to finish and orchestrate the score by using Sousa's fragmentary sketches, a piano draft, and one page of a completed band score.</mods:note>
          <mods:subject authority="lcsh">
            <mods:topic>Marches (Band), Arranged--Scores and parts.</mods:topic>
          </mods:subject>
          <mods:relatedItem type="constituent" ID="MODSP1">
            <mods:titleInfo>
              <mods:title>Piccolo</mods:title>
            </mods:titleInfo>
          </mods:relatedItem>
          <mods:relatedItem type="constituent" ID="MODSP2">
            <mods:titleInfo>
              <mods:title>Flute 1</mods:title>
            </mods:titleInfo>
          </mods:relatedItem>
 ...
          <mods:relatedItem type="constituent" ID="MODSP23">
            <mods:titleInfo>
              <mods:title>Drums</mods:title>
            </mods:titleInfo>
          </mods:relatedItem>
          <mods:identifier type="local">100010802</mods:identifier>
          <mods:location authority="marcorg">DLC</mods:location>
          <mods:recordInfo>
            <mods:recordContentSource>IHAS</mods:recordContentSource>
            <mods:recordCreationDate encoding="marc">030421</mods:recordCreationDate>
            <mods:recordChangeDate encoding="marc">030508</mods:recordChangeDate>
            <mods:recordIdentifier source="IHAS">loc.natlib.ihas.100010802</mods:recordIdentifier>
          </mods:recordInfo>
        </mods:mods>
      </mets:xmlData>
    </mets:mdWrap>
  </mets:dmdSec>
  <mets:fileSec>
    <mets:fileGrp>
      <mets:file MIMETYPE="image/tiff" ID="F1" GROUPID="G1">
        <mets:FLocat xlink:href="http://lcweb2.loc.gov/natlib/ihas/warehouse/patriotic/100010802/0001.tif" LOCTYPE="URL"/>
      </mets:file>
      <mets:file MIMETYPE="image/tiff" ID="F2" GROUPID="G2">
        <mets:FLocat xlink:href="http://lcweb2.loc.gov/natlib/ihas/warehouse/patriotic/100010802/0002.tif" LOCTYPE="URL"/>
      </mets:file>
      ...
    </mets:fileGrp>
  </mets:fileSec>
  <mets:structMap>
    <mets:div DMDID="MODS1" TYPE="sc:scoreObject">
      <mets:div TYPE="sc:score">
        <mets:div TYPE="sc:page">
          <mets:fptr FILEID="F1"/>
          <mets:fptr FILEID="F13"/>
        </mets:div>
        <mets:div TYPE="sc:page">
          <mets:fptr FILEID="F2"/>
          <mets:fptr FILEID="F14"/>
        </mets:div>
...
        <mets:div TYPE="sc:page">
          <mets:fptr FILEID="F12"/>
          <mets:fptr FILEID="F24"/>
        </mets:div>
      </mets:div>
      <mets:div TYPE="sc:parts">
        <mets:fptr FILEID="F72"/>
        <mets:div TYPE="sc:part" ID="PART1" DMDID="MODSP1">
          <mets:div TYPE="sc:page">
            <mets:fptr FILEID="F25"/>
            <mets:fptr FILEID="F26"/>
          </mets:div>
        </mets:div>
        <mets:div TYPE="sc:part" ID="PART2" DMDID="MODSP2">
          <mets:div TYPE="sc:page">
            <mets:fptr FILEID="F27"/>
            <mets:fptr FILEID="F28"/>
          </mets:div>
        </mets:div>
...
        <mets:div TYPE="sc:part" ID="PART23" DMDID="MODSP23">
          <mets:div TYPE="sc:page">
            <mets:fptr FILEID="F69"/>
            <mets:fptr FILEID="F70"/>
          </mets:div>
        </mets:div>
      </mets:div>
    </mets:div>
  </mets:structMap>
</mets:mets>