Adding a validation constraint in Symfony2 based on the existence and value of a form field via Symfony's form events. Good idea (best practice?) or dirty hack..?

LoginOrSignupType.php

<?php

namespace T7\Bundle\OAuthBundle\Form;

use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Validator\ExecutionContextInterface;
use Symfony\Component\Validator\Constraints as Assert;

use T7\Bundle\OAuthBundle\Entity\User;

class LoginOrSignupType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('email')
            ->add('password')
            ->add('new_user', 'choice', array(
                'choices'       => array(
                    0 => "I have a password.",
                    1 => "I'm new!",
                ),
                'multiple'      => false,
                'expanded'      => true,
                'mapped'        => false,
                'property_path' => null,
            ))
        ;

        $builder
            ->addEventListener(
                FormEvents::PRE_BIND,
                function(FormEvent $event) {
                    $data = $event->getData();
                    $form = $event->getForm();
                    if (isset($data['new_user']) && $data['new_user']) {
                        $form
                            ->add('email', 'text', array(
                                'constraints' => new Assert\Email(),
                            ))
                        ;
                    }
                }
            )
        ;
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class'      => 'T7\Bundle\OAuthBundle\Entity\User',
        ));
    }

    public function getName()
    {
        return 'user';
    }
}