[terryp]

Actually, I do write functional tests and I have been for a long time. However, I'm not a developer ... I'm a failed developer and figured I could best continue to develop if I had a QA title. The ammo that I always could use for functional tests was that they reveal a different dimension of wrong than a unit or integration test. And when you have unit, integration and functional tests development speeds along at a pretty rapid clip.

[tartley]

An interesting aspect of the "different dimension" is that functional tests actually demonstrate that your application works. If your website is down after deploying a well-functionally-tested application, then it's often because of something beyond your control, like an Amazon outage. Functional tests give you the confidence to deploy often, or automatically, even after bold refactoring, because you can prove that the app works, which unit tests or integration tests cannot do.