For many types of equipment, applications, or conditions, using a mechanical seal is really the only effective sealing option. This decision focuses on the safety or environmental issues associated with leakage.
A mechanical seal, and more specifically a double mechanical seals ,should always be used when the pumped fluid presents a safety, health, or environmental hazard. Packing cannot be 100% leak free. Even when leakage isn't visible, harmful vapors could be escaping. This is also true for single mechanical seals.