Since this title can be read in two different ways, I’ll state: James H. Fetzer is a defender and not a critic of the idea. He has done, or assembled, results of the most extensive investigations ever conducted of the Kennedy assassinations (there were two of them, after all, and in neither case does the official story stand up to criticism). These results blow the Warren Commission Report out of the water. Obtain Assassination Science: Experts Speak Out on the Death of JFK (1998) which he edited.
Professor Emeritus Fetzer has also written and edited numerous well-received works in the philosophy of science, cognitive science, research into artificial intelligence, critical thinking, and all the cognate areas, all with major publishers. Why his own critical thinking skills should mysteriously desert him when it comes to conspiracy claims is, or ought to be, a complete mystery.
So where does the topic stand now, in 2021? If you read mainstream news, you might come away with the impression that being a “conspiracy theorist” is a mark of irrationality. My conclusion, which I suspect Fetzer would share, is that conspiracy theory / theorist are nothing more than weaponized phrases. They are signs, written by apologists of dominant narratives, of points of view or lines of thought we are not supposed to go down. They open doors those with wealth and power would rather remained closed and locked.
So is there merit to “conspiratorial” thinking? Go here, learn, and perhaps gain a little wisdom (the love of which being what philosophy was supposed to be about).