This is something that is touched upon much. TDD fits library writing like glove because the developer naturally thinking of the public contract of its componen

This is something that is touched upon much. TDD fits library writing like glove because the developer naturally thinking of the public contract of its components. This not to say that Application Code doesn't need the same approach but its a lot harder for developers who don't have their design hats to think of pieces of their functionality as contracts of behaviour that contribute to a feature or requirement. To your point to think of in terms of the life of their code. I think the top comment links to an excellent talk on TDD and how it is often misapplied.