I also think your point about generating defective children is important. I'm personally baffled by the idea that it is OK to slice, dice, and mutilate an embryo if its going to be killed but not if it is going to be left to grow. The idea that letting something live is a problem seems somewhat perverse to me.
While I no longer practice an active faith of any kind, I am greatly bothered by those who I perceive to be pursuing what I call a "technological messiah." By this term, I mean any technical advance that people believe will make life suddenly wonderful. For instance, an inexhaustable, inexpensive energy source would seemingly solve so many problems and make life much better in many ways. I'd welcome that advance, but I think its power to make us truly happy is vastly overrated. I fear that some people will believe that they can make human life much better by having only "genetically improved" babies. While I would welcome a cure for many birth defects, I don't believe that eugenics in a lab coat today will produce any more good than it did when practiced by racists in the 20's and 30's.
A second level of discomfort is that some parents might see "genetically improved" children as a way to compensate for negligence in raising children. Regardless of how "genetically perfect" a kid might be, bad parenting is still a way to make monsters.
Again, thanks for some interesting thoughts.
WFTR
Bill