Why was this removed from the ParseVariables method in the Email class for 2.4...
foreach(array('from','to','subject','body', 'plaintext_body', 'cc', 'bcc') as $param) {
$template = SSViewer::fromString($this->$param);
$this->$param = $template->process($data);
}
It was handy being able to have dynamic variables in the subject line, etc.