The really important thing is that you have to give enough exposure to the darkest thing of interest in the scene to make it register on the film (expose for the shadows). Development affects the shadows more or less the same regardless of extent (OK - extreme underdevelopment will underdevelop them too, but the shadows are more or less unaffected by the degree of development). The highlights are affected by development, however, and overdevelopment caused them to block up badly on older films. That's where the "develop for the highlights" came from. The situation is not so bad with modern films, but sticking to the adage still makes your printing easier. The zone system uses development to control contrast, which is a related but quite different thing. Since you can't avoid the "expose for the shadows" part and the zone system uses adjustments in development, confusion was bound to happen.