By repeated invocation in debate, contrived scenarios nudge audiences' intuitions in favor of torture in the real world, even when the real world scenario bears only the faintest resemblance to "ticking nuclear time bomb." It means that repeated debates about torture that feature ticking-time-bomb scenarios tend to
worsen rather than
improve lay audiences' ethical decision-making with regard to torture.
To see how debate can backfire, consider a currently-near-universal ethical proposition: it is unacceptable to torture innocent children. The contrived scenario for debate: there's a ticking nuclear time bomb hidden in an American city. The man who has the critical information can hold out against physical torture for the next few hours until it detonates. But he can be compelled to confess in time if you force him to witness the torture of his 5 year old niece. Imagine that the scenario has both options debated, repeatedly, in lecture halls, on op-ed pages. It's even depicted in fiction like 24 to persuade by storytelling rather than logic.
It doesn't matter that the world's actual torturers have never encountered a ticking nuclear time bomb, much less the improbable other circumstances that led to this dilemma. Just witnessing sober, respectable persons arguing the "in favor" position for innocent-child-torture -- no matter how dryly or laden with caveats -- is going to tend to weaken norms against innocent-child-torture. So will seeing the tactic succeed in fiction. (It doesn't really matter if excellent arguments against innocent-child-torture are also aired in the same venues. Practically everyone started out against it, even if their reasons were visceral rather than logical.) Some percentage of people, hopefully small, is going to come to "see the light" in favor of this contrived scenario's utilitarian calculus.
The really pernicious part: as with the vanilla ticking time bomb scenario, many of the persuaded will misapply the contrived scenario to justify real world torture. "The torturers know that an attack is imminent, and torturing this man will save a million lives" will get watered down by homeopathic magnitudes to "the torturers suspect that an attack is going to happen at some indefinite time, and torturing a child related to this man might save 20 lives if the torturers' guesses are correct." The effect on the body public of debating contrived scenarios that could justify atrocities is not to improve the public's reasoning or shore up their pre-existing intuitions. It's to erode their actual ethical judgment, to make them more open-minded regarding atrocities against children.