A regulator's casings should not use non-metallic parts for what reason?

Using non-metallic parts in a regulator's casing poses durability concerns that can compromise safety and performance. Regulators are critical components in propane systems, as they control the pressure and flow of gas. Non-metallic materials may not withstand the high pressures and temperatures typically associated with propane gas applications, leading to potential failures. Such failures could result in leaks, which pose serious safety hazards, including fire hazards or explosions. Therefore, utilizing durable, metallic materials ensures that the regulator can reliably operate under the demanding conditions of propane systems, maintaining the safety and integrity of the overall installation.

The other options relate to aspects that are less critical when it comes to safety. While cost efficiency, weight reduction, and ease of installation are important considerations for overall design and practical application, the paramount concern in the context of regulatory components is ensuring they are built to last and can withstand operational stresses.