Wouldn't it be hard to make a Night Lord ally with others?
In Black Crusade, although you are all playing Chaos, and will have the same general agenda, each player will likely have their own particular goals within that, some of which may be contradictory to what other players might want .. especially if you have players who are aligning towards opposed factions (Khorne/Slaanesh or Nurgle/Tzeentch).
For example, the players aim might be to corrupt a particular hive world. The Nurgle player would think plagues a wonderful way to do this, the survivors being led to Grandfather Nurgle. The Slaaneshi player would obviously rather corrupt the hive nobility, leading their decadent excesses into the worship of Slaanesh. Both ways could satisfy the overall goal, corrupt the world, but they're likely to be mutually exclusive.
That being said, the Slaaneshi player could concentrate on the nobles while the Nurgle one starts with the lowest ranking underhivers, and maybe they both get a bit of what they want.