A facility for adjusting a number of servers assigned to server pools for performing certain work types on the basis of unmet service needs in a work processing facility. Servers may include service agents, both human and robotic. A server assignor and a corresponding server assignment method may each be employed in a work distributor or an automatic call distributor (“ACD”) to conditionally adjust server availability in server pools. The server assignor compares a composite preference value for a work type against each server's threshold value for the work type. When the server assignor determines that the composite preference value is greater than or equal to a server's threshold value, then the server assignor indicates that the server may be included in the server pool for that work type. Each server has preference values and threshold values for different kinds of work. The magnitude of a preference value represents an affinity for the work type. The server's threshold value represents a reluctance to perform work having that work type. The server does not normally receive work for which the preference value is less than the threshold value, unless the server assignor determines that the composite preference value exceeds the server's threshold value for that work type. The server assignor and corresponding server assignment method may compute the composite preference value from a number of user-selectable inputs and utilize a number of user-selectable functions.