The appeal is obvious: it grants authority w/o requiring competence. He doesn't need to understand the subject. Just imply he understands you. Instead of refuting your conclusion, he explains why someone like you would need to believe it. Especially handy if the argument's going poorly. Facts are stubborn, logic is humiliating, evidence keeps existing independently of his self-image. So Reply Guy™ changes games.
Can't beat the point? Pathologize the person.
Can't play the hand? Play the man!