The thing I rarely see brought up in discussions of automated testing is longevity. If you have a team of developers and the codebase is going to be around for

The thing I rarely see brought up in discussions of automated testing is longevity. If you have a team of developers and the codebase is going to be around for a while, you need to grow tests over time so that the someone doesn't accidentally break something they weren't aware of and so that you can refactor your code with confidence that you're not breaking existing functionality. Sure, in an ideal world, everything is isolated by clean interfaces and encapsulation, but even in that ideal world, you still have a complex system that produces sometimes unexpected side effects (the emergent behavior of a software ecosystem).