What the hypothetical does is this: It forces people who normally refuse to answer, or who answer untruthfully (not you, by the way), to answer this question: "Is the destruction of a minute-old embryo morally equivalent to the destruction of a five-year-old child?"
Some say no. Some say yes. Neither is necessarily wrong. But, unlike the stupid pro-choice denial that an embryo is both human and alive, this approach to the discussion at least identifies an argument that is legitimate and not-automatic.
You made the point earlier that this doesn't mean that it is morally acceptable to destroy the embryo. Quite right. But it is a particular step toward a particular line of inquiry, one that is more fruitful than 99.999% of the abortion debate that takes place here in PWI.