> Im not convinced you really gain that much in terms of reducing complexity or attack surface by using these technologies.

In the case you mention (which I posted), integration of some kind offers functionality that would be otherwise impossible to get--single sign-on which lowers friction of user acquisition. SAML is one alternative method, but from my experience it has some sharp edges too.

Of course, whenever you're integrating a third party solution, you need to do so carefully.