Feast is a decent levelling/regearing tool in the right hands, though I agree that its PK utility is minimal.

Black circle, though... you're insane.

I'm hesitant to make a sweeping statement like: "In the hands of a good player, there's no deadlier cabal power in the game", but... yeah, it's in that class for me. Give me the choice between BC and insect swarm on a necromancer and I'll have to seriously think about it... and then I'll probably pick BC.