Drupal 7, login, "form_build_id" not used? I write some functional tests for my website.
I have this test :


go to login page
post username/pass/form_build_id
check if the user is connected


In the test script, my form-build-id is "hardcoded". So, I run the connection test severals times and...The user connects each run (whereas the form-build-id does not change)

I don't understand, why the form-build-id is ignored ? 
I check with my web browser, each time I refreshed my page, the form generate a new form-build-id.
As drupal_prepare_form() notes:


  The form_build_id is used as key to cache a particular build of the
  form. For multi-step forms, this allows the user to go back to an
  earlier build, make changes, and re-submit.


More than anything else, form_build_id is for maintaining the form state through page requests. As you can see in some of the core processing functions (e.g. drupal_build_form(), drupal_rebuild_form(), &amp; drupal_process_form()), form_build_id isn't required for form processing.

In some instances (e.g. multi-step forms) breaking the form_build_id would break the behavior of the form since there is previous data to retrieve, but for the login form there's no state needed to be preserved.