Quote - but also I'm sure I did not equate Shader Mixer to the Poser material room because in mixer you can redefine the attributes (nodes?) that appear in the main shader (and hence what the end user has to input in the surface tab ui) ... while in Poser you have to start with the predefined "PoserSurface". So it's still not quite the same in the context I was thinking,  and I'm not sure what else to equate it to.  Any suggestions? :)

no, it's still the Material Room.  so.....
this is actually a problem i see a lot in different contexts.  people get caught up with what something is called instead of how it functions.  the Poser Surface gives you slots that automatically do stuff for you and have names so that you can put them away easily.  think of it sort of like a spice rack that takes containers of absolutely any size.  just because it says oregano, doesn't mean you can't put cinnamon there.  and if it says, "Alternate Spice," or "Good Taste," you can put whatever you think should go there.

you can put absolutely anything you want into Ambient, Alternate_Diffuse, Alternate_Specular, and Translucence, and outside of a few special situations, the results will be the same.  diffuse internally applies the diffuse node, and specular internally applies the specular node, but the rest just show what you give them (well, Translucence does some crazy things in certain situations, but let's not get into that because it's kind of complex).  and because Poser's nodes are flexible, and the base nodes are there, like different types of diffuse and specular, you can put anything and any combination of anything anywhere.  there's absolutely no more flexibility in saying your main surface is user defined and saying you provided user defined input to your globally defined root node.

the only thing making the base user defined would do is give slots user defined labels.  imho, that's of debateable use.  in general, when you give a whole populace flexibility in terms of labeling, what you have is a mess of questionable usability and little consistency. but folksonomies vs. taxonomies is a huge usability discussion.