A valuable employee is one who can contribute to the goals of the organization and is a good fit for the job.
Most managers will tell you they don’t have trouble identifying a valuable employee. However, most managers incorrectly define "valuable." They look only at the employee's productivity, willingness to do whatever is asked, and potential for promotion. Managers may cast aside valuable employees because they question management policies and procedures; they may label these employees "difficult to work with" and chastised them for not being team players.
Objectively, every employee should be "valued," but not every employee in every job is "valuable." Some employees are working in the wrong place; they are simply not the right person for the job; they don't have the qualities, characteristics, and ability to contribute. However, they may be valuable elsewhere in the company. The fact that they are not "valuable" in this job does not mean I shouldn't respect or appreciate (value) them as a people.
Managers require maturity to understand the difference between "valued" and "valuable." They have difficulty assessing how valuable an employee is for many reasons. Some organizations have poor job definition, leaving managers to say, "How can I assess whether employees are a good fit if I don't know what I'm trying to fit them into?" Managers may have personality conflicts with some employees who "push their buttons;" they are never able to assess them fairly. Finally, some managers have poor management skills; if they do not have the skill of "assessing and coaching," then they are unable to judge an employee's fit for a job.