Its is currently impossible to put an i18n key in a javascript variable because the script must be surrounded by a <comment> tag like :

<script type="text/javascript">
  <xsl:comment>
    var i18nVariable = "<i18n:text i18n:key="THE_KEY" />";
  </xsl:comment>
</script>

This is impossible because the <i18n> tag can't be inside the the <comment> tag.

And sometimes this kind of variables is the only way to translate a text (especially if the text is in a js file in the skin because there is no resources_dynamic directory).