shaders: Why is the second normal transparent layer disabled by default?

As described here, to configure a transparent coating material with normals for both layers in Unreal Engine 4, it is first necessary to enable this function in the project configuration:

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Why is this not enabled by default?

More specifically: will enabling have an impact on the overall performance of all materials?

(I can imagine it by adding an additional computational thread to each material, even when they do not use the Clear Coat Shading Model).