It depends upon what sort of conversation they expect to have. IFF they're open-minded and ready to be challenged and reconsider their conclusions, then it's a conversation worth having. You might even learn something useful about how they reached their conclusion in the first place. If they're closed-minded and nursing a self-delusion, your chance of being heard is next to nil, and the effort would be wasted.
I can't tell you how to predict which ones are which.