I'm currently reading an interesting tutorial about GLSL shaders in OpenGL and I'm understanding why you need to know about about 3D graphics when shading compared to the plain use of built-in features. If a shader really replaces the OpenGL functionality, you have to do all on your own - that's where knowledge and skill comes in. I have to admit that I never quite got why all games requires shaders though most of them only use so basic effects and graphics so that it isn't really needed at all. Of course, render quality is definitely a concern but probably doesn't justify the additional development time needed to completely reinvent the old stuff.
Well, I'm just learning right now, so 60% of what I'm saying can be simply false. However, I don't think that I'll need shaders that soon (except when having to use it during my internship or something like that...). All I'm having in mind can be created with simple standard OpenGL stuff, so I'm happy about it! Don't make too big leaps if not required.