I was reviewing the codex again last night. The only benefit I can see to it is if you wanted to run it as an ally to CSM, so you would have a CSM/Black Legion list.

I also guess it allows anyone, (i.e. Chaos Lord or Daemon Prince) not just abaddon to take chosen as troops.

But I think the main benefit is allying it with CSM. I was thinking of running CSM/Black with Typhus and Abaddon for