Abortion is abortion is....?
I can't say with scholarly precision, but it sounds like a thought experiment meant to test the limits of one's opposition to abortion. Fair enough, but it has no counterpart in law; at least none that I know of.
Wikipedia is hardly the most scholarly resource, but it doesn't mention any state which restricts abortions only to cases of rape or incest. And by "restricts," I mean outlaws it entirely (which would contravene Roe v. Wade and probably violate the Casey "undue burden" standard). Given the Casey decision, in fact, I'm not sure limiting abortion to cases of rape or incest would ever pass muster, and I don't know of a state which has tried to do so.
So what is the argument about? Money.
This issue came up most recently (prior to Rep. Akin's remarks) in 2010, when Akin and Paul Ryan tried to restrict federal funding for abortions, and were worried about any exception based on rape or incest. They weren't trying to ban abortions (they knew that wouldn't work); they were simply trying to ban federal funding for abortions (it's money that matters!). The people pressing for this restriction were worried about how federal money was going to be spent.
I don't think that worked out too well, either.
So now we have to ask: where's the beef? The idea of the validity of the exception already cedes ground to the anti-abortion side which they don't currently control. Any discussion of the exception is, in fact, a sop to the "middle ground," to those who are not devoted to one side of this issue or the other. It had to have started as a way of sounding more reasonable than the opposition. But as a point of concern for pro-choice advocates, it's pointless: Roe and Casey aren't going to be overruled because of the rape/incest exception. If anything, it has abortion opponents creating absurd arguments which only serve to underline the poverty of their claims.
I mean, when Rush Limbaugh says your anti-abortion argument is groundless.....