With customer surveys, you can survey a sub-set of the customer base as long as the sample is representative of the overall population. However, with employees, factors other than validity must be considered. Employees are much like families and inclusion should be an important part of the company culture. When an employee survey is deployed to a sample of employees, those who are not asked to participate may feel undervalued or insecure about their future at the organization. When all employees are asked to participate in the survey, it reinforces that the survey is designed for organizational improvement and with sufficient representation from each workgroup, department, etc. you can understand key issues and perceptions within those units.