If agent B does proclaim the argument X in favor of Z being true and you believe that X is not convincing then this still gives you new information about agent B and the likelihood of Z being true. You might now conclude that Z is slightly more likely to be true because of additional information in favor of Z

B believes that X argues for Z, but you might well believe that X argues against Z. (You are considering a model of a public debate, while this comment was more about principles for an argument between two people.)

Also, it's strange that you are contemplating levels of belief in Z, while A and B assert it being purely true or false. How overconfident of them.

(Haven't yet got around to a complete reply rectifying the model, but will do eventually.)