A binary (or trinary if the game includes neutral) moral system is boring mechanically.

Just look at the Infamous series; the developers talk about all the decisions you get to make through the story but in reality you're only making ONE choice at the beginning of the game to be good or evil and then every other choice is beholden to that because you need to farm goodboy or badboy points to get the strongest abilities. There's no chance for nuance or the ability to craft your characters morality.