Block cache?

Why do we do not cache the block? I mean its like a menu that doesnt change every 2 seconds.

/**
 * Implementation of hook_block_info();
 */
function taxonomy_block_block_info() {
  $blocks['taxonomy_block'] = array(
    'info'  => t('Taxonomy Block'),
    'cache' => DRUPAL_NO_CACHE,
  );

  return $blocks;
}