Why should shutting an engine down by turning off the boost pumps and closing the airplane's fuel valve be used during emergencies only?

Shutting down an engine by turning off the boost pumps and closing the airplane's fuel valve should indeed be reserved for emergencies due to the potential negative impacts it can have on the fuel system's service life. This method can create inconsistent pressure and flow characteristics throughout the fuel system, potentially leading to wear and tear or damage over time. Regular operation may not require such drastic measures, and using them outside of emergency scenarios could lead to frequent maintenance issues, thereby shortening the lifespan of components designed to manage fuel delivery and pressure.

The other choices do not directly relate to the fundamental mechanics of the fuel system in a manner that consistently affects its longevity compared to the wear and stress that can arise from emergency shutdown methods. Understanding the operational intent of the fuel system and its components can help flight crews make informed decisions during both normal operations and crises.