One thing I learned early on from working at a college computer lab in my late teens is that people very often don't even see certain elements on the screen. The post I responded to made it sound like people are deciding not buy because they consciously evaluated the button color and didn't like it.

Maybe people didn't notice the button when it was a different color. An A/B test couldn't tell you that but having watched a lot of people using unfamiliar software, that seems a lot more plausible.

If the point of a website is to communicate then A/B tests are a good way of making sure that you're communicating effectively.