* but what if you work in an environment where you can't afford to test every single little thing, and where business models, let alone requirements change constantly (ie. a startup)?*
Constantly changing requirements are exactly why you want tests. Not only do they help you find regression bugs when requirements change, but they help tremendously in writing code that is easy to maintain. If you can't test a piece of code, it's a red flag that you can't easily change it either.
Furthermore, I don't buy the notion that writing test code adds significant time to the development process. I do not have metrics, but I would argue that a good test suite actually reduces total development time, since the time writing tests is more than offset by the time saved testing manually and fixing bugs later.
Constantly changing requirements are exactly why you want tests. Not only do they help you find regression bugs when requirements change, but they help tremendously in writing code that is easy to maintain. If you can't test a piece of code, it's a red flag that you can't easily change it either.
Furthermore, I don't buy the notion that writing test code adds significant time to the development process. I do not have metrics, but I would argue that a good test suite actually reduces total development time, since the time writing tests is more than offset by the time saved testing manually and fixing bugs later.