[poorpointofview]

If an employee does NOT publicly speak about her or his experience she or he is providing the company with less of an incentive to value his or her experience there.

The public image of a company has value to a company and if the company knows that an employee can speak out about the employee's experience at the company they will have more of an incentive to ensure that employee's experience is pleasant because to do otherwise would harm their public image and their hiring abilities.

By being silent you are creating information asymmetry(the employer knows more than the employee) in the job market which contributes to people making less informed decisions. Less informed decisions leads to inefficiency.

The mature thing to do is to inform the public to ensure that competition between companies does not stop.

By being silent you are throwing wrenches in the wheels of regulated market capitalism.

[Cyndre]

This one can be taken even farther and be applied to the "don't discuss your salary". My opinion has always been that this rule exists so that companies can continue to underpay employees.

[creamyhorror]

Great post. Information asymmetry leading to market inefficiency (and poor outcomes for the info-poor side) is how I think of it too.