I predict here and now, that at some point in the future Cabals will be multi faction. I can understand why they want to start off enforcing the divide between the secret societies, but it in the lore as it stands, and in gameplay, it makes little sense in the long term.
Players want to form affiliations with who they game with, not be straight jacketed. Choosing your faction does affect gameplay and story, but not who you team with in pve, in what is primarily and mainly a pve driven game at the moment.
Remember, you can team with the other societies. The previous enmity between the societies is not abated, but now moderated by an agreement that they all have a greater enemy to fight. So it makes quite reasonable sense that as the usual prohibitions are relaxed in the face of new and urgent priorities requiring inter society co-operation, that bonds of friendship would form between society members. This is a reasonable assumption within the game lore, and within rl, between the actual players themselves.
Re PVP, if i'm correct... You will have Faction versus Faction pvp, same as is planned for now. You will also have Cabal versus Cabal, which will be multi faction on both sides.
Limiting the ability of players to form cabals, provides no actual usefulness, limits gameplay and is a bit inconsistent with in-game lore. As FD has said before, to be fair, devs are often not totally consistent with their own game lore. You can team up and plan a mission with the opposing factions, but you cant "do" that in a regular manner? So you can team continously with your Illuminati and Templar friends... but you cant also pool resources and/or share a nickname for the team? I dont get it... What is a Cabal/Guild except a group of people/avatars that regularly team together and pool resources as a unit? The basic requirement of a good cabal/guild is teaming together. Thats allowed already.
If the prohibition is to be kept consistent with the lore, you should be prohibited from teaming with the same outsider/enemy faction member more than a set number of times. Team too many times with the same members of opposing factions and your loyalty to your own faction should be questioned. "We dont mind exceptions to the norm, we all need to fight the greater evil, but let us not forget, they are still our enemies" or somesuch in game statement reinforced with in game mechanics along the same lines is needed. That would provide sufficient ingame justification for the prohibition of multi faction cabals.
But thats not been stated so far, or anything like it, in terms of limts of who you can team with.
So, to sum up. I can understand why a game would launch with mechanics and systems that enforce percieved divides. But in the long term, i see the prohibition of multi faction cabals being lifted.