If you've only worked on projects with nailed-down requirements, you're probably not working on the sorts of projects most HN people face. The requirements change because the world changes, or our understanding of it. That's the nature of the startup. Stable codebases serving stable needs don't need as much refactoring, that's true. And in those cases units might be a waste of time. But for those of us (the majority, I'd wager, at least around here) who work on fast-moving, highly speculative projects, they are an absolute godsend.