Probably the fact that you can only trust a Khorne Berserker as far as the distance to his chain axe is. Or as likely as a Necron-Blood Angels alliance, or the idea that Grey Knights would slaughter Sororitas and bathe in their blood! ... also, I believe there has been an episode where Daleks and a certain Time Lord had a common interest at the beginning - it's just that you know that at one point one will turn against the other, and that's what would happen when allying with the followers of Chaos as well. Not that I want to disagree with you there, but Khorne isn't exactly everything Chaos has to offer. It could be in the interest of any other ruinous power, since they don't restrict themselves to bloodshed and melee combat - although I will admit that this is not of interest to one who follows the Bloodgods teachings.