My general rule: If you are planning on some sort of reuse (API, library, protocol, etc) a test spec is a very good idea if only to hash out practical use of t

My general rule: If you are planning on some sort of reuse (API, library, protocol, etc) a test spec is a very good idea if only to hash out practical use of the interface and identify caveats in the interface spec. Once released, the test spec is great for answering the question "did I accidentally break something that used to work?" thus vastly speeding up the bug-fix process for underlying components or widely used components. If you are writing an end-user app, or a service that will be used non-programmatically (such as a GUI or web app), then a test spec is not required and probably a waste of time as things will likely change too quickly.