There are two arguments I see being made by those who find alignment unuseful:
1) There are alleged connotations and parallels being drawn to real world groups when we define a race as evil.
2) it makes boring story telling.
Both of these I refute.
1) D&D as was, was a game based on fantasy tropes, drawing from the rich veins of mythology, legend and fantasy fiction inspired by these tropes. The audience understood this. Gods in the game were real, they had a presence and impact. Races (used in the original sense of the word as a synonym for species) created by the gods. Good, evil, law, chaos were objective, tangible meta physical aspects that affected the world. It was part of the world of shared imagination, asking us to believe in dragons, magic and an identifiable good evil. Some items could only be used by evil people of great power, others only of the truest heart.
Creatures could be born into evil, designed as such by the gods. Yes they had intelligence, they were self aware, but they had not the will nor inclination to change (at least without great difficulty). It was a humano centric game and vision. Only the human race, upstart, young could break these bonds, their diversity their strength. The other creatures, not humans in funny ears or green skin suits, but beings of a wholly alien mindset. There were no parallels being drawn to real life groups and the negative language used to describe them applies to all of us. The orcs are savage, barbaric, bestial, rapacious because they are the worst of us. Drawn from myth and allegory, they are the monsters we become in war, our base crueller instincts. Informed by Judeo-Christian influences on understandings of good and evil, they are evil. Born into this sin, manifestations of the chaos wilder lands and borderlands against “civilised” society.
Now certainly, there is an argument to be made for moral relativism. Yet consider the male lion. On taking over a new pride, he will kill the young of the previous male. There is no good or evil ascribed to this, it just is. Give that lion intelligence and self awareness, viewed through the prism of human experience and understanding, I think many would throw aside relativistic values and describe that as evil.
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2) It makes boring story telling? Does it though? Let’s unpack that. Some of the most enduring myths (informed by the different cultures’ religions and understanding of what was good and evil) and modern fantasy are good and evil.
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Without alignment, you are free to make your morally grey world, and explore orc cultures though! Great, this was original and daring in the nineties and early 2000’s as media explored this concept as a counter cultural deconstruction of the traditional evil orc. Hell, this is what made Eberron special, it was a deconstruction of all those D&D tropes. The problem is, if default D&D is Eberron, what is special about it any more? Generally cool Orcs have become the cultural norm, expectation and trope now. You are doing nothing daring and new with that. Surely it would be more subversive to go back to the creatures of evil now would it not?
Yes a morally grey world can be interesting, complex and mixed with great ideas. It does not make it a superior form of story telling. It does not make a classic good vs evil tale boring, it’s certainly a less sophisticated method of story telling, but one that has resonated with humanity for millennia.
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Those that have called for their removal recently, using either points 1 or 2 have shown a startling lack of respect to the game, their fellow players and ignorance of the wider mythology, literature and cultural history that has informed this game.