The Beta 5 patch notes included this:
We installed a spine algorithm into the AI and it will no longer beg for mercy after every turn
Which does seem to be working, since they no longer plea for mercy after every turn. But they do still ask for peace after they're dead, not just on every turn:
That's the Drengin leader asking for peace. You probably can't see him because he's under his desk, dead. He is no more. He has ceased to be. Bereft of life, he rests in peace.
Besides the screen shot, I have a saved game if needed to prove that the Drengin are no more.