“So am I understanding it correctly that the Principled BSDF shader is usually all you need to use as a shader node, except for rare cases, since the other ones are built into the Principled BSDF?”
That’s pretty much it, yes. Also Mixing Shaders is slower (to calculate) than using a Principled Shader.
But sometimes you want more control over separate components (ie. Shader Nodes), or break physics, because it looks better in a specific case, then you can (will have to) use Mix Shaders instead…
…and, for instance, in 4.0, the Sheen in the Principled is different from the actual Sheen Shader Node (Cycles only).
