They can have a psychological impact.
Biological WMD's could possibly have a psychological impact that will go far beyond their actual effect. The very thought of exposure to a biological agent may possibly cause many people to panic.
Biological WMDs can tie up resources.
Some biological agents can be a hazard for lengthy periods. The use of these agents may require tedious, time-consuming, resource-intensive decontamination and monitoring of facilities before they can be returned to service.
Defense may be difficult.
It is very difficult for civilian government agencies to prepare for biological terrorist incidents. While most civilian agencies have some kind of hazardous material or HAZMAT response teams; in the event of a biological terrorist incident, these teams are likely to be challenged beyond their capability in terms of human resources, and equipment.