I do wish they had made Chaos allies of convinience with use. I have a Chaos army in addition to my DE, and according to the Black Crusade RPG, it's not uncommon for the Dark Eldar to co-operate in some way with human rengades, including Chaos worshipping ones. They might buy slaves off of them, sell them weapons and other services, or just raid and enslave them, whatver makes the most profit. Dark Eldar are ultimately quite pragmatic.