As for how useful is mixing shader is, I don’t know. I know before the Principle BSDF the mix shader node was used a lot to create materials. As for me, I use the principle BSDF node and the only time I use the mix shader node is as a mask between two different principles shaders. Note: mask is 0(Black) and 1(White) image texture that switched between 2 shaders and is plugged into the fac. Black will be the top shader input. White will be the bottom shader input.
