This same question came to my mind, I was planning to go with this approach by creating a unique Material Blending Layer and decide the channels I want to blend with static switches. This way the shader complexity would be reduced as it does not calculate the “branches” generated by the switches, however, and I am not really sure about this, as far as I know static switches generate a lot of shader permutations (I think up to 2^n, n → number of static switches in the worst case) that the engine needs to compile when releasing a build, and cause long loading times or ingame stuttering.
However, as I say, I am not 100% sure if Material Layers are handled this way by Unreal and I would appreciate so much if someone shed some light on this topic.