Do we still need "Drupal Users Table Name" setting on CMS Database Integration screen?
There are scripts in extern/*.php (like extern/ipn.php) which fire up CiviCRM without the CMS bootstrapped. I believe the plan is to replace them with callbacks (like civicrm/payment/ipn/%id) which eliminate instances where CiviCRM is bootstrapped without the CMS. (Some links here, but don't think there's a JIRA issue on it?)

So: Perhaps in the few places where CiviCRM doesn't have Drupal bootstrapped, eg new user create during PayPal IPN on successful event rego? (I made up this case as a potential place CiviCRM might need it, and not be able to deduce from bootstrapped CMS.)