As a starter, I'm extremely sceptical of modifying the genome of embryos (as I hope was clear from my comment). Like you said, there's plenty of defects that can have advantageous sides in addition to the negative sides. My point is that the only way this could even conceivably be ethical to do, is if it was used to remove diseases.

I believe there's already a procedure they do to remove a disease carried in the mother's mitochondria. I heard about a rare disease from my dentist the other day that causes people to never grow any teeth. I'm saying that if this is done, then the only ethical thing to do is use it for population-wide removal of severe diseases with no known positive sides, and even that can be dubious (as you point out).