It only shows that the person is good at doing things that they find interesting. I am a largely self taught man but I have learned a lot of things in school that I am grateful for learning but probably wouldn't have spent a lot of time learning on my own. Probability & Statistics and DSP are two that come to mind off the top of my head. I probably would have done them to a cursory understanding and not spent the countless hours in class and studying that it takes to get a deeper level of knowledge. That is not to say that I wouldn't have been learning though; it just would have been in a different area.

Learning subjects and doing well in school shows a potential employer that you are indeed capable succeeding in doing things that do not interest you. This is a skill that is important to employers.