I would extend that even to custom embryos as well. Yes, you have a smattering of disease causing genetic variations that cause disease when they are homogeneously expressed. However, we are far from full comprehension or total knowledge about all the genetic and epigenetic factors at work in human genetic expression. Those disease causing genes may well convey survival benefits to the species under certain edge cases.
See for instance, delta-32 ccr5. Without HIV and smallpox, it would be hard to imagine why a defective receptor protein would be desirable. Since we dont know what future diseases may manifest (as we dont have crystal balls), wholesale elimination of defective genetic variants via designer embryos is potentially shooting the species in the foot.
Is this a serious ethical snarl? Certainly-- where do you balance "I dont want to suffer from cystic fibrosis you cunt!" with "Do you know what all the consequences of removing that genetic variation from the genome are, and are you willing to be responsible for it?"
Right now, we just dont have enough of the picture to safely remove purely deleterious variations, as we cannot fully quantify that they are indeed wholly deleterious. As our knowledge improves, and techniques improve, my position will change. To me, we are at the stage where people thought it should be routine to remove tonsils. We know better today, and only remove tonsils for specific reasons.