Celebrim said:
Contrary to some claims, I don't think a universe were nothing is alien and everything is basically human is more complex in its conception than one where alien things 'outsiders' are in thier basic nature alien.
So 'outsiders' must either be incapable of choice or they are basically human? I submit that this is a false dichotomy.
Fiends in D&D are, by and large, neither fallen celestials nor are they particularly anthropomorphic in their mindset. They do not think or act like people, but the lack of free will is not one of the many ways in which they are unlike people.
They are alien because they are the personifications of various philosophies, reified evil, chaos, or law. They are ideas with minds and teeth. Not only do they exist to propagate the ideas they represent, but they
are those ideas.
As a result, both "fallen" celestials and "risen" fiends are impossibly rare - certainly far rarer than a mortal making a similar change in views. But they do happen because on occasion ideas can be twisted or corrupted, conflicted between means and ends or afflicted with the curse called Love. For a fiend to rise or a celestial to fall, their very composition is altered by the tension between what they are and what they are beginning to believe. And there is no reason for the fallen to outnumber the risen, other than general pessimism.
Your point that divorcing the material from its mythic roots gives it less emotional power has a lot of truth in it, but the theology you describe is so esoteric that I think it adds nothing to the visceral emotional response such characters incite. Making fiends incapable of redemption doesn't make a game feel more mythic because this isn't a part of most peoples' theological or literary background, and it's not the universal view (and I think there's more than a little sophistry in the argument you recounted in any case - angels decided to fall with full knowledge of the eventual result? Really? Lake of fire and final destruction after Armegeddon and all?). It's such a counterintuitive bit of theological wankery that I think, if anything, it detracts from the experience.
Don't get me wrong, Celebrim, I thought your post was fascinating to read, but I don't agree with your conclusions.
The sort of mythology that adds to the experience isn't theological esoterica, but evocative names and powerful themes - the Fall is one such theme, and as a result including fallen celestials as NPCs can be a great addition to the game. But redemption is an equally powerful theme, and that shouldn't be forgotten.