A handy example of how to hide non-required fields from a form - in this case, on a profile2_regpath altered user_register_form

drupal_form_hide_nonrequired_fields

<?php

function MYMODULE_form_alter(&$form, &$form_state, $form_id){
  if ($form_id == 'user_register_form') {
    $fields = array_filter($form['profile_PROFILE_TYPE_NAME'], "_filter_required_fields");
    foreach ($fields as $field_name => $field) {
      $form['profile_PROFILE_TYPE_NAME'][$field_name]['und']['#access'] = false;
    }
  }
}

function _filter_required_fields($field){
  if (is_array($field) && isset($field['und'])){
    if (!isset($field['und']['#required']) || $field['und']['#required'] == false){
      return $field;
    }
  }
}
?>