I hope to start an interesting discussion with this post. It's a thought that's been on my mind for a while now. I'm trying to stick to 2e (or at most 3.5e) canon, by the way.
Night hags are an interesting lot. Unlike the various types of hags of the Material Plane, which are nothing more than monstrous giant females with a natural penchant for magic and a sweet tooth for human children, Night Hags are denizens of the Outer Planes. While they seem to be closely linked with the Grey Waste, they are also associated with dreams (namely nightmares), secrets, and trade. Even though everybody dreams, the nature of dreaming is a big mystery. Sure, some talk of a Plane of Dreams, but its relation to the Great Wheel is about as well understood as that of the Ordial. Anything definite about it is as likely to be screed as incredibly misinformed. The Night Hags, however, are intrinsically linked to mortal dreams, in both their manners of subsistence and reproduction. Secrets are by nature mysterious. They are not just knowledge, but knowledge shared by a few, which seems to increase its power, like water being pushed through a smaller nozzle. The Night Hags' power seems to stem from their hoarding of secrets, and the physically weak (in appearance to mortals and in comparison to fiends) Hags have managed to thrive despite a dangerous environment and the center of the domineering empire of 'loths being planted in their own backyard. Finally, their purpose seems to be in trade. They have entrenched themselves in the cycles of the Lower Planes by being indispensable. While the 'loths are playing the two warring factions of the Blood War by selling mercenary services, the Hags are (through a more passive neutrality) trading various minor necessities for all sorts of knick-knacks that won't be missed until somebody finds the secret for putting them all together into something useful. And the Hags know an awful lot of secrets.
The Night Hags also have interesting relations with other races. It seems pretty certain that despite similarities, the Hags of the Wastes are not in any way related to the hags of the Material Plane. Nobody's quite sure why this is certain, but it must be. After all, the one are outsiders while the other are just monsters. However, outsiders that can mutate a Yugoloth prince into a completely different creature, an act on par with the Baernaloths that would make even the Obyriths writhe with jealousy, would presumably have little problem experimenting on their dream-producing little cattle for reasons of their own. Closer to home, they have all sorts of relationships with the fiends that surround them, usually through trade, but they are particularly close with the equine Nightmares that roam the Waste. There is also that odd relationship with the Fey through some connection between the deities Cegilune and Titania, and both races are associated with dreams and, to a lesser degree, mortal children.
Night Hags seem to very individualistic, though similar in their practices and inscrutable goals, so it is no surprise that they range the spectrum in terms of power. Some are little more than shack-dwelling hermits, happy to herd their larvae to to the next bazaar. Others are plotting megalomaniacs, trying to unravel the puzzles of the Planes. A few are even after godhood. Indeed, it is odd that a race of outsiders should have their own goddess. While many outsiders have deity-level rulers, few have a patron power, even if the relationship is one of mutual hate. There's also the matter of Malagard, an oddly aligned Baatezu (nee Night Hag) countess with deific aspirations. While the word on the street is that she was a dupe and has since been disposed, it is no small matter to dupe a Night Hag, especially one that has climbed so far. Some whisper that she planned the whole thing, and even the new Nine seem somewhat uneasy on the subject. Finally, there is a certain Baba Yaga, a hag associated with the Slavic Primes, though whether she is a deity (of life and death, if you would believe it), a hag of legend, or something else entirely is uncertain. She does seem more like the Night Hags than the lowly Material Plane types, and by all accounts at odds with Cegilune.
So, feel free to discuss any of the issues raised. Of course, everything is speculative. I am particularly interested in the trio of powers - Cegilune, Malagard, and Baba Yaga. After all, the Rule of Threes for hags is called a Coven.
EDIT: I don't know where the dice rolls are coming from or how to stop them.
From the long-abandoned "Darkest Grey" forum comes my contribution to this line of thought, which is interesting to me too: the Sisterhood of Lenience, a hag cult. After all, Cegilune may be a racial goddess, but a deity will take whatever worshipers she can get.