Actually, the purpose of the "rape or incest" gambit is not to suggest that is when most abortions happen, but rather to put the "abortion is murder" position holder in a tricky position. If abortion in indeed murder just like shooting someone on the street, then how can abortion in cases of rape or incest be permissible?
The "Abortion is Murder" proponent must either somehow either soften the claim or hold his/her ground and take the unpleasant but consistant position that these abortions are also murder. Unplesant as it will result in babies with severe birth defects, and also unplesant as it in effect allows a rapist to force a woman to carry his child.
The "abortion is murder" proponent can counter this gambit as far as "cases of rape" go, but the incest part is a bit dodgier, except that it largely becomes a euthenasia type argument where the difference of opinion bases itself in basic presumptions people hold about the nature and purpose of life.
So in the end this gambit, like most rhetorical gambits, only bear fruit or score points when used on the unwary. [/B]