Are you kidding? From that guy's perspective, of course it's smart! It is a deft rhetorical maneuver! He's absolutely delighted to get to play the "tone" card. When you get to be the one who plays the tone card, you demonstrate that YOU know what a "serious discussion" should be, and what it shouldn't.

He doesn't have to worry about her response. In his mind, he's already won, because by saying that he undoubtedly feels that he has invalidated her entire side of the argument.