I've always felt that the rape/incest exemption adopted by anti-choicers burns a hole their arguments and is, above all, a telltale sign of their true intentions: They are not concerned with the right to life and protection of 32cell embryos, they are merely trying to maintain pregnancy as punishment.
Beyond the frailty of their argument, there are severe logistical problems with trying to uphold a law that outlaws abortion except in certain cases. Here is an excellent discussion on the topic by Dadahead (posted at The liberal Avenger). Just a taste:
Those who hold this position should be asked one important question: how, exactly, would you discriminate between those pregnancies which are, and those which aren't, the result of rape? That is, how would the state go about determining which cases are legitimate exceptions?
None of the options seem plausible. One possibility is to simply ask each woman how she got pregnant. If she says it was because of a sexual assault, then she's allowed to abort; if not, then she can't. For obvious reasons, this is not a very good policy; an abortion ban under these circumstances would be utterly toothless, requiring only that a woman say she was raped. While this would be an odious indignity to impose upon women, it wouldn't do much to prevent the abortions it was supposed to. Years ago, the state of Pennsylvania would pay for abortions if they resulted from rape, requiring only that the woman claim that she had been raped. This apparently resulted in a 'wink-wink' kind of practice among clinic workers who knew that an affirmative answer to the rape question was necessary to get state funding. There's no reason not to expect that this phenomenon would become even more common if the rape claim was necessary not only for funding but to procure the abortion itself.
And speaking of feeble arguments, here is a perfect example of how anti-choicers don't base their opinions on logic and reason and in fact don't really think their arguments through. Watch the video on Goddess Musings (via feministe).