most labels on anything are arbitrary.  the labels the Shader Mixer user would choose would be not only arbitrary, but idiosyncratic and personal.  for instance, most bagginsbill followers use the Alternate_Diffuse channel for what it sounds like: a particular way to shade a surface that's different from the primary diffuse.  it usually includes specular, but that doesn't stop Alternate_Diffuse from being a pretty decent label for that purpose.  

you could probably do the same in D|S, even without the Shader Mixer.  you could plug an image into Reflection Color, and i doubt it will behave any differently than if you plugged it into Ambient Color.  the key is to stop looking at labels and start looking at how the application actually works.