This post is not about the relative merits of either side in the abortion debate. Instead (and this should be really obvious from the title), I want to discuss why it's a stupid argument. More to the point, I want to look at some of the reasons why it's nearly impossible to have a productive discussion on this topic.
The problem is not just that the public (or at least media) discussion of the issue has been oversimplified to point of absurdity; or that the two sides are so utterly polarized that there's no common ground at all; or even that neither of those positions actually fit the way most people think - though all of that is, to some degree, true. It's the structure of the debate itself, which conflates two very different questions and concerns.
What it breaks down to (in the oversimplified, polarized, make-people-fighting-mad presentation) is this: "Killing babies is bad" vs. "Abortion should be legally available". And then, of course, we have people shouting them at each other, as if they were actually talking about the same thing:
"Killing babies is bad."
"But abortion should be legally available."
"But killing babies is bad!"
"But abortion should be legally available!
"But KILLING BABIES is BAD!"
"But Abortion SHOULD BE LEGALLY AVAILABLE!"
...And like that.
Until you start breaking the argument down, there's no way to make progress. And, to be honest, I think a lot of our politicians prefer it this way. The issue generates a lot of righteous anger, and gets people to vote. If it ever went away, they'd have to find some other way to motivate people, and that takes work.