It changes things because the arguments from both sides are disingenuous and inconsistent. A pro-life person arguing against abortion because human life begins at conception shouldn't have any exceptions for rape or incest. Once the argument is framed around when life begins, all other things become irrelevant. And a pro-choice person arguing in support of abortion because "human life hasn't begun yet" is making an argument that can't be grounded in anything besides how they feel. They are attempting to make a scientific argument that can't be rooted in science.

But morality is subjective by its very definition. This makes it perfect for framing the debate because all the arguments from both sides are subjective as well.