Thats a fair comment, but in a similar vein; do you trust your own ability (as manager I presume) to evaluate the work output and business value of the employees? Or otherwise do you trust a / your manager with this?

One thing that is true, is often times the manager has more information about the various reasons for certain choices and goings on in the business, and thus is in a better position to evaluate people.

The question then is just, how do you make sure that manager do not discriminate based on soft indicators (personalities match / clashes / favouritism etc.).