[ddfreyne]

I believe it boils down to a matter of discipline.

If you say that you never randomly modify code in order to make it work (according to the tests), then that is great, and that is how every developer should work. However, my experience is that people who lack discipline (or face a looming deadline) tend to not take their time to properly reason about code and instead rely on the test suite to tell them whether code is right or not.