To me, abortion is about life of an embryo (foetus) versus... what? The thing that's on the other side of the scales is very important.
Speculation about future children doesn't fit in the equation, in my opinion. Genetically, "now a child" is always better than "later a child", other things being equal. The difference comes only when the "now" child (the one whose abortion we're discussing) and the "later" child (the one who wouldn't exist if the "now" is not aborted) are somehow of a different quality.
Suppose a women is pregnant after a rape. Would you want to pair your genes with the ones of a rapist? More importantly: how will the mother feel about the child? Will she be able to love and raise him/her in the way a child needs? The way a human being with half your genes deserves from you?
It may sound really harsh and unloving to judge the "quality" of the children before they are born... but it's reality. I don't want to be a child who is hated by her mother.
Another situation. What if, due to circumstances, the mother is "now" not able to care for a child, but later on maybe she is? Then wouldn't it be better to choose for the child who would get the better care and thus the better chance to survive and reproduce? Genetically, of course, the answer is yes. But we (especially we, the humans) are not slaves of our genes. We can thwart them - a good example of this is anticonception. I think there the things become more gray, discussion is possible, and the mother's choice should be taken into account.