The only time tests aren't the right thing to do is when you can guarantee that you will completely trash that system at some point and that code is never going

The only time tests aren't the right thing to do is when you can guarantee that you will completely trash that system at some point and that code is never going to see the light of a production machine. Most code gets refactored out of it's initial state, and that's where your tests offer huge benefits. I can say that I didn't truly appreciate the full value of tests until the first time I had to do a massive refactoring of a well tested app. Being able to change core fundamental components of a system and trust that everything will Just Work™ when you're done is an awesome feeling.