What's being debated is whether it's right to make experiments who's consequences a person who can't consent to them has to carry.
And this differs from having children the old fashioned way how?
That is the crux: every child born is a genetic experiment today. We hope that it doesn't end up with too many defects. We hope that it's born with eyes (I know someone who wasn't). We hope that it's born without any non-lethal developmental defects (I know a couple of people who have them, caught and fixed by surgery before they became fatal.) We hope they won't develop Type I diabetes (like several of my friends have)... and so on.
Every one of these things is a crap-shoot, and everyone who has kids today is performing an uncontrolled genetic experiment every time.
To claim that this process is necessarily going to be made worse by adding some human intelligence to the mix is problematic. The claim that there is no conceivable therapeutic benefit to engineering certain classes of genetic defect out of the germline is disingenuous, unless you think everyone everywhere for the rest of time is going to have good gene therapy available to them while growing up, which is insane.
To claim "we can't predict what the effects might be so we shouldn't do the research" is bizarre: if we don't do the research, admittedly mostly on animal models, how will we ever know what the effects are? Once we've done the research, we will have a high confidence in the effects, just as we do with any other therapeutic intervention. We all know that iatrogenic disease is a major cause of death and suffering, so wringing our hands about germline modification as if it was unique in that respect is at the very least strange and at worst deeply hypocritical.
All power gets used, and it's understandable that people should be concerned about how the power to edit the human germline will be used. The possibilities for abuse are considerable. If we are ever able to create Brave-New-World style designer slaves, happy in their subjugation, we probably will, for some value of "we".
But the debate should be about how to use this power wisely, not whether we should develop it at all. Someone will, and it's better that that happen out in the open than in some secret lab in $EVIL_NATION or funded by $EVIL_BILLIONAIRE.