I had a need want to have my field groups formatted in fieldsets with legends rather than divs with labels as I believe the former is more semantically useful (ie I know that a fieldset is for grouping similar fields, but I don't know what a div groups).

I would have hoped that I would be able to achieve this with a .ss file, but it seems that /sapphire/forms/FieldGroup.php returns HTML directly using the function FieldHolder().

I was able to adjust the HTML code there to get the output I desired, but why isn't it in a template?

It is not in a template as we found early on having each form field in its own template caused a large performance lost, so as a workaround for that its in the php. You could use your own custom subclass of the field to override the template it outputs

I needed to do this for a form which renders dynamic fieldsets from a db query and ended up extending FieldGroup, here is the code if anyone is interested. Very straight-forward and quite specific (needed a table layout) but you can do whatever you want:

<?php
class PublishFieldGroup extends FieldGroup {
    function Field() {
        $fs = $this->FieldSet();
        $spaceZebra = isset($this->zebra) ? " $this->zebra" : '';
        $idAtt = isset($this->id) ? " id=\"{$this->id}\"" : '';
        $content = "<tr>";
        foreach($fs as $subfield) {
            $childZebra = (!isset($childZebra) || $childZebra == "odd") ? "even" : "odd";
            if($subfield->hasMethod('setZebra')) $subfield->setZebra($childZebra);
            $content .= "<td class=\"publishFieldgroupField\">" . $subfield->{$this->subfieldParam}() . "</td>";
        }
        $content .= "</tr>";
        return $content;
    }
}
?>