Not every APEX developer should be a Plug-In developer.
The APEX developer uses the Plug-In a Plug In developer has created.
Following photo shows the pictograms we used during our presentation @ OBUG APEX SIG :
During the demo we showed our little green man while we created and elaborated the plug-in via the shared components.
When we implemented the plug in in a page, we showed the pictogram of the little red man.
As you can see the red guy brings different elements of the puzzle together: standard components (red) and a plug-in (the green piece).