plugin field widget looks broken when there's only one plugin available

When there is only one plugin of the type used by the field, the field widget just shows the field label and nothing else. This looks completely broken.

Poking at the code, it looks like what's going on is that the widget sets a value, which doesn't output in the form:

    $element['select']['container']['plugin_id'] = array(
      '#type' => 'value',
      '#value' => $this->getSelectedPlugin()->getPluginId(),
    );

This is bad on two counts:

a) it's really bad UX to show something that looks broken. If there is only one option that must be selected, show it and disable the form element
b) it's NOT the only option anyway, as if the field is not required the user could select nothing.