I think to begin with it's futile to try to cover all relevant behaviour in tests as you introduce new code. Some basic functionality tests will do fine to prevent anyone from completely breaking the code, as well as providing fair documentation as to what the developer expects the code to do.

However, I think regression tests are useful. Once you find a bug and fix it, the things learned from fixing the bug can be expressed in a test, to prevent similar bugs from happening again. In such a case, the test documents an unexpected corner case that the developer was unable to predict.