Alternative title: Why it's ok for Skulltaker to be down for some Neshi luvin'.
A few editions ago of Warhammer the traditional rivalry of the Chaos gods was shook up. Your army could suddenly include Slaaneshi daemons happily cavorting around angry Khornate bloodletters, and Nurgle daemons could let rip next to Tzeentchian horrors who wouldn't bat an eye. This new interpretation is often described disdainfully by veterans as the new “fluffy bunny Chaos”, or something to that effect. I want to show you why it actually makes MORE sense than the old rivalaries.
In short, because it is more chaotic.
Nowhere is the new standing officially portrayed that all Chaos daemons get along with all other Chaos daemons. For brevity I am going to stick with Khorne and Slaanesh. Any Slaaneshi army is capable of allying with any Khorne army, it does happen. That does not mean that all Slaaneshi daemons get along with Khorne, it doesn't even mean that those particular Slaaneshi daemons like red 'meat'.
Each daemon has a modicum of personality, generally the more powerful the more complete a personality they have though even at best it does not tend to match a regular mortal. A friend put it this way: “Each daemon has a desire to bolster their deity, conflicting with a personal drive to be there towards the top, gaining power for themselves, a rather right wing take on strength of the deities daemonic society.” Which I think is a really funny and depressing commentry.
Chaos often fights against itself and it is not unheard of for a Khorne daemon to ally with a rival gods daemon in order to destroy the army of another Khorne daemon. It could be due to insult, “Graaahhh Skulltaker said I fight like a girl! Blood for the Blood God!” or it could be to further their own standing within their god's followers, or it could be just cause.
The end result is little structure for even general alliances that their gods make. Khorne and Slaanesh have a truce today, that may stop a lot of the fighting but there is always going to be some conflict. Slaanesh and Khorne might be at war here, but over there they are joining up to take on Tzeentch or to destroy a snooty Elf army. Khorne probably couldn't care less, lack of respect for any rules/truces probably goes hand in hand, even if they don't agree on specific cirumstances where the truce may or may not apply.
At any one point there would be alliances, wars and truces and it would constantly shift. It is suitably, chaotic.
That being said the gods themselves would give some structure to the standings. There was one story where Khorne was winning a war with Slaanesh who ended up suing for peace. A truce was called so we do know general truces and wars (and probably alliances) can happen.
That word, GENERAL, is the key. While today most (but not all), Slaaneshi daemons may be working with Khorne, tomorrow is a new day and most (but not all) Slaaneshi daemons could be warring with Khorne.
In the meta sense this is also good. It gives players choice and allows the daemon armies to be balanced around all of the models, rather than bloating codex/army book with choice to ensure that a player can make a balanced army ready for all comers four times over. It is also the 40k galaxy and the fantasy world in a microcosm, the background is intentionally created to facilitate almost any army (player) being able to reasonably engage any other army (player).
I did arrive at this conclusion independently but I believe that stance is pretty official now. I mention it though because I still see the odd snobbery in regards to the new take. I will just ask, what sounds chaotic to you, the mess I have described above, or a world where all Khorne daemons always, and consistently, get pissy at Slaaneshi daemons.