I have a recurring discussion with my friend James, regarding test-first development. He believes is that writing tests first is like writing an outline for an academic paper or novel. And sometimes you don't know where the story is going to end up when you start writing. In the past, I've always countered that with the liberal-arts-major argument that you should know what you're going to say before you say it.
However, he makes a good point. Even when you have a goal in mind, sometimes the path to that goal takes unexpected twists and turns. If you write your tests presuming a specific order of steps, they'll become obsolete when the real story doesn't follow the outline.
So here's a different response: the tests are the story. The production code is merely a footnote.
The ultimate goal is satisfied users, and the tests are development standins for those users (even if the "user" is another piece of code). As you write your tests, you should be thinking not of the needs of your mainline code, but the needs of those users. As your understanding of those users changes, your tests will change. Some will be deleted.
You could call this wasted effort, but that's misleading. It was necessary effort, because you didn't have a perfect understanding of the users' needs. The real question is: was it better to explore the plot using tests, or to simply write some mainline code and hope that it did what it should?