Agreed. For me it brings to mind Keith Olbermann's "Worst Person in the World" segment, which is not a good sign. It's not so bad if it's done with a wink in a one-off blog post, but I wouldn't want it to stick or be used more widely.

"Worst Argument in the World" seems like a particularly inappropriate label here, because it's generous to even call these sorts of slogans "arguments." Saying "Abortion is murder!" or "Evolutionary psychology is sexist!" is, at best, a vague gesture in the direction of an argument. There may be a coherent argument somewhere in that approximate direction, but if all you're doing is attaching a one-word label ("it's murder!") and leaving the rest implicit then you're probably just talking to System 1 (activating emotions and associations). As a dismissive put-down of this tactic, "that's not even an argument" seems more apt than "that's the worst argument in the world."