CAS is definitely useful for internal authentication - you have a database of users and a pile of apps. It's really easy to hack CAS authentication into almost any web app, and you can write a custom server if you like.

It's significantly less useful for external authentication - where you want to your apps to auth against other people's user databases. SAML 2.0 is usually the standard for that, but as you say, it has to be configured properly on both ends.