Though I have never, and will never sign one of these agreements, I cannot agree with your statement that they "reduce[] overall market efficiency". First off, such a grand claim merits some citation of empirical or theoretical evidence. Secondly, there are a variety of ways that these agreements (which I do not support) may increase productivity, such as by increasing the employer's trust in the employee, allowing the latter to access confidential documents, and increasing the average period of employment, thereby giving the employee more time to accrue experience and improve productivity in a specific field and/or application.