Those who call themselves the Legion of Darkness would kill anyone outside their order with a slightest thought. Lying, cheating, stealing , and killing; whatever it takes to get your precious power. No one trusts Legion. If you do you get screwed over. Look at Lycanthor. Reduced to simple healing and sanc for Lath and Quazark whenever they fought the Knights. You would think Legion would be grateful for the help. When Lycanthor refused to help him kill Zap, Legion kills him.
If I was in a cabal or not in a cabal. I would much rather take my chances with Mystic companions, or Warlords, or Justice. With Legion I know I'm going to be screwed over. Knights will help anyone and expect nothing in return. If you were not in Legion, who would to choose Blackstaff?