How can I show all symbols in an SVG file?

You can open file in Inkscape and choose menu Object -> Symbols (or press Ctrl+Shift+Y) an choose "Current document" in drop-down menu.

One file per svg file? Tedious is right!

Only slightly less tedious (but perhaps you could use a script to generate this, though):

<svg>
   <use  x="50"  y="50" xlink:href="icons.svg#icon-nextstep-compare" />
   <use x="100"  y="50" xlink:href="icons.svg#icon-arrow-down" />
   <use  x="50" y="100" xlink:href="icons.svg#icon-arrow-down-double" />
   <!-- etc, etc -->
</svg>

---

UPDATE

You could even generate the above with the magic of an xslt stylesheet. :-)

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0"
                xmlns="http://www.w3.org/2000/svg"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:svg="http://www.w3.org/2000/svg"
                xmlns:xlink="http://www.w3.org/1999/xlink">
  <xsl:template match="/">
    <svg>
      <xsl:for-each select="//svg:symbol">
        <use>
          <xsl:attribute name="x"><xsl:value-of select="position() mod 10 * 20"/></xsl:attribute>
          <xsl:attribute name="y"><xsl:value-of select="floor(position() div 10) * 20"/></xsl:attribute>
          <xsl:attribute name="xlink:href">icons.svg#<xsl:value-of select="@id"/></xsl:attribute>
        </use>
      </xsl:for-each>
    </svg>
  </xsl:template>
</xsl:stylesheet>