David Autor has a fantastic post on Vox on economics of labour market intermediation. Labour markets are imperfect and they are kept that way by interest of recruiters and employees. In the end both loose but still there is enough weight in status quo.
Let us consider an employer and a prospective employee. There is no way for the employer to know the employees performance in previous organisation. Therefore the metric is salary drawn currently. Now if, by any chance, an employee accepts a job with lower salary then his salary growth is lower! This appears unfair - but its actually a manifestation of inefficient market at work. The only way this situation corrects is when imperfection on other side - i.e. you have a desperate employer.
As per my experience the intermediaries actually twist this situation to the benefit of the employer. Hence most of the time employees distrust HR managers and intermediaries. That is the reason salaries are secret. Often even though companies treat everyone fairly - employees still feel distrust.
In sum, there needs to be two areas of research in this. First - does this behaviour actually benefit the employer at all. Second - does this distort the labour markets to an extent that it impacts the economy. My intuitive guess is no on first count and yes on second.