I agree with this post. I very rarely consider using a service's API without a client library. Taking the load off authentication and marshaling is incredibly valuable in my development workflow. Granted, sometimes simple REST endpoints are OK if they are public / don't need complicated authentication.

I think the major benefits of client libraries is the ability for a developer to stay within their programming language / business domain and not stray into the fun of HTTP and JSON.

Curious how much burden this makes for providers though and what people have done to streamline the testing of the various client libraries with different versions of the service.