Correct usage of add_rewrite in plugin

Following this post
http://ww.wp.xz.cn/support/topic/208830?replies=6
I came up with this code:

function wftp_rewrite() {
  global $wp_rewrite;
  $wp_rewrite->flush_rules();
  add_rewrite_rule('wftp/([^/]+)/?$', 'index.php?wftp=$matches[1]','top');
}
function wftp_vars($public_query_vars) {
    $public_query_vars[] = 'wftp';
    return $public_query_vars;
}
add_filter('query_vars', 'wftp_vars');
add_action('init', 'wftp_rewrite');

But that doesn’t work. However, based on this post http://codex.ww.wp.xz.cn/Custom_Queries, the following does work:

function wftp_flush_rewrite() {
  global $wp_rewrite;
  $wp_rewrite->flush_rules();
}
function wftp_vars($public_query_vars) {
    $public_query_vars[] = 'wftp';
    return $public_query_vars;
}
function wftp_add_rewrite_rules($wp_rewrite) {
  $new_rules = array(
     'wftp/(.+)' => 'index.php?wftp=' .
       $wp_rewrite->preg_index(1));
  $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}
add_action('init', 'wftp_flush_rewrite');
add_filter('query_vars', 'wftp_vars');
add_action('generate_rewrite_rules', 'wftp_add_rewrite_rules');
add_action('parse_query', 'wftp_init');

Can anyone explain to me why I can’t use add_rewrite_rule in this way, since it seems like it works in this post:
http://danmarvelo.us/older/2007/5/30/custom-rewrite-wordpress-add_rewrite_rule-add_rewrite_tag/

I don’t understand why I have to add directly to $wp_rewrite…
Thanks for your help!