I’m about to make a rather controversial statement now: “Developers who use purely inside-out avatar of TDD, .i.e. use unit tests to drive their development, are really using tests to validate their design and not so much to drive the design”.
The reason I think this way is because in my experience, developers who drive development using unit tests already have a mental model, whiteboard model, CRC card model, etc. of their design. Unit tests really helps them validate that design. (This is clearly not same as BUFD (Big Upfront Design). In this context its micro-design, design of a small feature not the whole system). Of course there is some amount of driving the design as well. When developers find out that the design they had thought about does not really fit well, they evolve it and use unit tests to get feedback. But as you can see the intent is not really to discover a new design by writing the test first.
Is this wrong? Is this a sign that you are a poor developer? Absolutely no. Its perfectly fine to do so and it also works really well in some cases for some people. But I want to highlight that there is another approach to influence design as well.
In my experience, using an Acceptance tests helps better with driving/discovering micro-design. When your test is not in the implementation land, when its in the defining-the-problem-land, there are better chances of tests helping you to discover the design.
While TDD is great at driving/discovering micro-design, Prototyping is great as driving/discovering macro-design.