Perhaps we should add a couple of useXXValidation() methods on the form, to turn this on / off?
$form->useJavascriptValidation(false);
$form->usePHPValidation(false);
If we did that, the best thing would be to have a FormField::includeJavascriptValidation() method that was called on each Field as part of the form rendering, if javascript validation was selected. It would be a bit of a refactoring, but it would put the responsibility for deciding whether to include javascript validation in the hands of the Form object, which is more appropriate than in the individual form fields.
If we can make a patch for this on trunk, then Dio5 / Blackdog can apply it to their own branches before the next release.