Just-in-time provisioning of Drupal user accounts based on SAML attributes (configurable). Automatic role assignment based on SAML attributes (configurable). Dual mode - support for traditional Drupal accounts and SAML-authenticated accounts at the same time (configurable).
Drupal offers a variety of solutions to address multiple sites with shared and separate content. As with all things Drupal there is no right solution - just options for your specific needs. Evaluate your goals and make decisions based on them; you may find that Domain Access is a better fit for your content editors or a multi-site setup is much easier to manage for your system administrators.