They could have put a couple of pages in explaining why these races would ally so the stupid people get it, they did it for 'why would Demonhunters fight non-demons'.
I am excited about seeing some Xenos terrain, I must admit. Perhaps I am being harsh and overly cynical, but I just can't shake the feeling that commercial origins, rather than gaming ones, underly these additions. They took Dogs of War out because they broke the game and the Warhammer ally matrix is based on two people's playing together so why change it? But hey, if it works for other people, enjoy the allies, hte beuaty of the game is my group can choose not to use it if we wish while yours embrace and modify it to suit your gaming.