Add non mandatory fields to user registration form
I have never encountered such problem before. But since you have such problem then I think the best way is to write a hook_form_alter. And set the required field that you don't want as mandatory to false.

function mymodule_form_alter(&amp;$form, $form_state, $form_id){
switch ($form_id)  {
    case 'registration_form_id': 
                $form['field_name']['und'][0]['value']['#required'] = FALSE;
            break;
                   }
               }


Hope this helps.