How do I determine whether the AI's comment 'this email is not professional enough' is useful?

First, ask what the email is trying to accomplish, then check whether the AI's assessment actually addresses that goal. If the aim is to convey information, you can verify whether the content is clear; if the aim is to maintain the relationship, show concern, or tactfully express disagreement, you cannot only evaluate whether the argument is direct and rigorous. The article uses the analogy of a basketball referee evaluating a piano performance to remind us that evaluation standards may fundamentally be mismatched. Therefore, the AI's comment can serve as a reference for revision, but you should verify whether the standards it applied match the actual needs of this business communication.