Irrelevant for me.
Explanation of monsters' nature and origin is the kind of fluff which I actually enjoy reading a lot, because it can give me inspiration for writing adventures and campaigns. But at the end of the day, I'm free to pick what I like, discard the rest and make up as much fluff as I want.
For instance, when I started gaming I was intrigued by the wacko cosmology of the great wheel and blood war. Two years later, I was bored by the wacko cosmology of the great wheel and blood war

Lately I've been assuming very different things, like Hades, Hell and Abyss being on top of each other: Hades be the entance to Hell where the souls of the damned are gathered, Hell being the eternal punishment for those souls, and Abyss being a place below Hell where mortal souls never go, but where devils or other outsiders may be sent as their own punishment.
In this new case, fine for me to say the gelugons are demons (to be honest, I do hate devils that look like insects, it makes no sense to me). Fine for me to say that Succubus are devils, but at this point I see no reason to keep the "devils are always LE" mantra. I like that Succubi tempt humans not because they want to gain a soul for hell/abyss/whatever, but "just because", which is quite CE if you ask me.
I'm not sure about Erinyes, but I seem to remember that they are spirits of fury and vengeance and have nothing to do with tempting the mortals, and maybe they are such in D&D only because of some designers mistake/ignorance?
