UncleArkie said:

Don't believe the DEldar offer anything to Slaanesh. They hate it too. They keep their spirits bolstered by inflicting pain - they don't give that pain to Slaanesh.

Kshatriya said:

The Dark Eldar don't offer anything, but by continuing to live like before the Fall they allow her to take it. From "The Torturer's Tale" (a story Asdrubael Vect tells to a human prisoner in an old White Dwarf. Google it if you want to read the whole thing):

"As time passed through an eternal nightmare of half-reality – for the emergence of the Great Enemy created the vortex known to you as the Eye of Terror, engulfing our oldest worlds – it became clear to our hero that She Who Thirsts was not finished with our people, her hunger would never be sated. She had a grip on our spirits and though temporally assuaged by the massive slaking of Her thirst during Her birth, She still needs to drink. Our lord-to-be felt Her thirst lapping at him and saw it in the faces of others, their essence being slowly leeched away by the Nightmare That Hungers."

"It seemed there was but one way of escaping Her and that was to flee their homes and leave the physical world behind forever. We came here, into the realm between worlds that we created to traverse the galaxy safe from harm. Here, the Great Enemy's grip is weakened, yet to our Lord's horror it was not wholly broken. He had bought his people time, a little instant of time but nothing more."

"Our founder looked upon the outside world, horrified by the beasts rampantly breeding across our domains. But then a thought occurred to him. Perhaps She Who Thirsts would drink others, as well as us. He sent some of his many warriors to capture a few of the man-things that had been spawned by an insignificant blue world in the western spiral arm. His best counselors and experts examined them and indeed these beasts, for all their crudity, still contained that vital essence of life, that spark of spirit that turns a fleshy vessel into a living thing."

Mjoellnir said:

Except that much of the Dark Eldar background changed when the new codex came out. Asdrubael Vect's story in The Torturer's Tale is no longer accurate (because Vect's backstory differs from his account of it there, and because the settlements in the Webway were already heavily inhabited by the cults of excess long before the final days of the Fall), and the matter of what they actually do with their captives has been clarified.

Dark Eldar consume the agony and terror of others, essentially devouring souls to replenish their own withered essence. They don't sacrifice others to She Who Thirsts as an alternative to their souls.

N0-1_H3r3 said:

Strange, I thought they actually republished the story when the new Dark Eldar were released, however, you still don't get what I'm saying. The Dark Eldar don't sacrifice others. Slaanesh slowly leeches off their souls and they replenish it from others. Essentially Slaanesh drinks from a bunch of bottles that is constantly being refilled. The bottles don't become empty at which point they would be thrown into the trash (dead) but Slaanesh still drinks more and more and more.

 Which just leads back to "please don't eat my soul", the replenishment is just another way of preserving the self which is what is important to the Eldar as a race, they are more like two factions than two races. 

UncleArkie said:

Mjoellnir said:

It's not "please don't eat my soul" for the Dark Eldar, it's "please take all you want from my soul, I'll just replenish it later with the soul of some tortured slave." The Dark Eldar allow Slaanesh to constantly feed on them while the Craftworlders lock her out. And with the constant feeding the Dark Eldar improve Slaanesh's chances of beating Ynnead to a pulp and devouring him. Which since he consists of the collected souls of the Craftworlders…. gnah, I'm already repeating myself.

Mjoellnir said:

You make it sound like there's willingness involved.

The souls of the Dark Eldar are being forcibly dragged away, though far slower than they would be if they dwelled in realspace. They don't have a choice in the matter - it would happen regardless of what other actions they took (the only ways out, demonstrably, are to bind yourself to a Soulstone… and that isn't guaranteed as soulstones can be broken, or to become one of the Harlequins whose souls are bound to Cegorach). They devour the essence of others to replenish their souls and prolong their existences, denying She Who Thirsts their souls in death.

In essence, between the revifification that many powerful Dark Eldar undergo should they actually die, and the devouring of the souls of those too weak to endure, the Dark Eldar themselves serve as a collective repository for the energies of their kin.

The Dark Eldar do differ from their craftworlder kin on a metaphysical level - their souls are atrophied, feeble echoes of what they once were, requiring the regular invigoration of imbibed torment to truly sustain them, lest they become little more than wretched husks devoid of sensation… but to regard their nature as some form of offering to the Great Enemy is (IMO) a serious misinterpretation of the text.

As you said, they do have the choice to bind themselves to soulstones. To make a real-world example (hopefully without sparking a political debate), Craftworld Eldar try to fight global warming by reducing emissions. Dark Eldar try to survive the consequences with air conditioning which makes the greenhouse effect worse. Sure, the Dark Eldar could happily live without having their souls drained off, just like no one likes to live at 50° C, but they are unwilling to go the hard way and limit themselves. The effects of the actions of both peoples should set them at odds.

I'd suggest those differences do put them at odds. They don't live together, or hang out for family dinners, for example, and Dark Eldar aren't welcome to just show up at Maidenworlds and set up shop or Craftworlds to hang out.

On the other hand, its like hanging out with the members of your extended family that you don't like. You may not want them over for supper, and you might not even like them, and you might make fun of them behind their back, but if the Russians invade Crazy Uncle Ross's farm, and I have an army, you can be sure I'm gonna show up if I can to help him.

Thats how I see it, anyway.