That's why I warned against conflating Tolkien with a roleplaying system like D&D. I don't think one can talk of a spectrum of morality in Tolkien. In fact, he reminds us that the highest and most virtuous are particularly prone to perdition. Also, what I think he meant with his commentary on orcs above, is that orcs, despite their thoroughly corrupted nature, are still a part of God's creation and subject to God's grace as such.
Yeah, and I think he was pussing out with this. His standard for irredeemable is ridiculous. If no one has the power to create except Iluvatar, and nothing Iluvatar created is irredeemably bad or evil, then no one can be evil. Which makes the word meaningless in the setting.
It's why I think that people should take his letters with a grain of salt. An author's finished works have received a lot more thought and planning that assorted scraps of notes put together in whatever apocrypha.
There is no equivalence between Good and Evil. Thus the distinction he makes between "bad" and "irredeemably bad", as the idea of an "irredeemably bad" creature would place it outside of God's domain. "Bad" is always limited in scope in comparison with "good".
Effectively the same as good and evil. Semantics. Call it "bad" or whatever, it is what it is.
The alignment system is a practical adaptation for people who don't want to start every game session with a theological debate. It gives everyone an idea of what a character should behave like and makes clear distinctions between characters, in order to bring variety. I don't think there's anything more to it.
There is a lot more to it.
D&D is a simulation. And as such, it needs trackable variables.
How do you decide what an evil deity is? Or a good one? How do you weigh the character of a player and decide whether or not he's worthy to wield a holy weapon or a sword of selection?
It's not about variety either. It's about good verses evil, and order vs chaos. Without moral differences, there can be no moral conflict. The alignment system was a way to quantify this, and to facilitate the creation of those conflicts. It was also a tool to help players play their chosen role and not just larp a murder hobo every time.
Maybe you could argue that the 9 alignment system is about variety vs the original system, but the alignment system itself is about more than that.
Edit: You are 100% right about the debate part though. Because that's what happens when you don't track variables and don't spell out rules for people. Even with the rules spelled out, there are people itt who openly announce they don't play by them because they're not "fun".