To answer that, let's ask two questions:

1) What if the Z/E/F is, or at some point in the development process prior to birth becomes, a "living person"? Is the outcome then relevant in determining the propriety of abortion? If it is, then can we not use that same standard to evaluate other living persons, like the profoundly retarded, medically fragile and severely impaired little girl who presents such a burden to family and society with little benefit to her in quality of life, or dear old Great Gramps with his advanced Alzheimer's?

2) What if the Z/E/F is, for all or part of the development process, no more "living" than your appendix? Is the outcome then relevant in determining the propriety of abortion? If it is, then can we not by the same logic set limits on any other elective medical procedure, making risk-benefit analysis a collective public health decision, considering that no surgical procedure is without risk and that there is ample evidence of abuse with resulting poor outcome? Can we, say, put an end to plastic surgery except for those with true need?

I would argue that:
1) once we determine life to have begun, then outcome is no more nor less relevant in the abortion debate than it would be in any other medical situation involving two interconnected persons, and the benefit or detriment to third parties (i.e. the later-born children you mentioned earlier) is never relevant in such circumstances,

and
2) until we determine life to have begun, it is a medical issue between doctor and patient, so the issue of outcome is nobody's business but the patient's and her doctor's, and the meddling outsider has no business sticking his/her nose into it.