Yes it is, because the rules expects us to have one objective definition. Without that requirement it doesn't matter if people disagree what "good" means. Their PCs can debate moral issues in-character like people do in the real life, without anyone having to determine who is "objectively correct."@Man in the Funny Hat : Fundamentally the questions alignment raises are difficult ones. If you ask people, "What does it mean to be good?", not only is this question a vexingly complex one, but it's not a vexingly complex one that most people haven't spent a lot of time thinking about. But as you note, everyone already believes that they understand anyway so they'll confidently try to answer it. This isn't just a problem related to rules text in D&D.