I recently had a case, when I was sending a e-mail to subscribers, but each of them had either email or user id filled-in.
I used following code:
$send_params = Array ();
$send_params['to_name'] = $email;
$this->Application->EmailEventUser('EVENT.NAME', $user_id, $send_params);
And I've discovered a bug, that when user have $id, but not e-mail empty recipient is added resulting Fatal Error during e-mail sending.
Ready for testing.