Celebrim
Legend
I don't understand this.
Aasimar are not all good. Tieflings are not all evil. But aasimar have the powers of the upper planes and tieflings of the lower.
And therein is my problem. What would it mean to have powers of the upper planes to draw on if you weren't good? And how would those powers be good if the one drawing on them wasn't? Why would heaven as it were be in opposition to itself? And if the powers of the upper planes were good, wouldn't drawing on them be good as well? And if drawing on the powers of the upper planes is neither good nor evil, then why would we call them good? So this only works if the upper planes and lower planes aren't really the thing we say that they stand for at all, but merely just "hats" that things where with no more meaning than say a flag waved by a nation. This conception of aasimar are not all good but they can draw on the powers of the upper planes only works if we only are thinking of aasimar or indeed the upper planes as being a sort of tribe, or ethnicity, or species and they aren't all good either. And indeed, I really think that's what people are thinking, as witness the lore of popular media like Diablo or Spawn where it seems like fundamentally that's true.
But if that is the case, than well, this is at best being lazy and name dropping something for its resonance that really has nothing to do with what you are doing - we're calling them angels and demons and talking about upper planes and lower planes as if they had significance to cosmic reality but really they don't. And at worst, this is just cultural appropriation of the most disrespectful sort.
It would be one thing to be saying that aasimar are not all good and tieflings are not all evil, and they have no particular powers to draw upon, because their mortal nature dominates over any other part of their parentage. But there is a lot of "cake eating" in the conception that they get some power of their heritage that itsn't wholly the result of their evil or goodness, but that they also get all the free will of their mortal nature. Which goes back to my comparison of playing vampires as supers in leather jackets and ignoring the monstrous nature of the creature entirely. Cake eating.