user_login_name_validate() uses isset() where other user login form validation handlers use !empty()

function user_login_name_validate($form, &$form_state) {
  if (isset($form_state['values']['name']) && user_is_blocked($form_state['values']['name'])) {

but in user_login_authenticate_validate():

  if (!empty($form_state['values']['name']) && !empty($password)) {

and user_authenticate(), which gets given that value uses empty() too:

function user_authenticate($name, $password) {
  $uid = FALSE;
  if (!empty($name) && !empty($password)) {

This creates an inconsistency which makes it difficult for custom/contrib modules to do form alteration around user login (eg http://drupal.org/project/email_registration).

On D7, there is also user_account_form_validate() which uses !empty().