You are correct a in that 24-bit PNG file that has transparency is technically stored as a 32-bit file.  The alpha information is stored in the extra eight bit channel.  I also see what you are saying about directing the A output to the Alpha input and the RGB channel to Diffuse input works.   

I have years of experience in other tools such as Blender's node editor and only have hours of experience using Shader Forge.  The output node in Blender is implied as RGB or RGBA if alpha available.  Forgive me for assuming the same thing here and not fully realizing the difference.  I can understand how the input channels of the "Main" node expect a more explicit split.

This still don't explain my main concern that caused my confusion in the first place.  The Textured 2D preview is showing nothing but black.  Texture Asset shows the preview correctly.  If I connect the output of the Texture Asset to the Texture 2D input and no operation was performed between the nodes, I expect to see the exact same preview.  Likewise if I load the exact same asset directly in to the Texture 2D node I would expect to see the image with the transparency.

That is an awesome idea!  

It would be even more awesome if someday in the future Shader Forge could also be used to create the .cginc file.  For example, creating a file with the custom lighting that would be used instead of Phong, Blinn-Phong, etc. in future shaders.  And it would be awesome if a group of nodes could be saved so they could be re-used.  (I am assuming the group would be stored as a .cginc file.)