In many cases genes correlate with this or that problem but not absolutely and yet people will go on and talk about them as though they do. In those cases we need to know more information. It could be that under the right circumstances having the gene will provide more benefits than not having it. In other cases we know for sure it's the gene itself that is the problem such as Huntington's Tay Sachs and Down's Syndrome. Once we can cure that we should.
Gene therapy, while not perfect, has come a long way since when it was first introduced. If it can continue to help aid in relieving genetic disorders, why would we disallow the use? The more it's researched and implements the better chance there will be for a greater level of success in the future.
The argument that gene therapy is playing God is a bad case of morality over mortality. A child has a defect that we can fix, yet we do not. This fix will keep the child from having a debilitating disease, defect, or mutation that can be avoided through gene therapy, yet we do not interfere. Why is that? Is this not directly relating to "do no harm?" Isn't failing to act in a good way just as bad as acting in a bad way?
The use of this science to correct genetic disorders is an excellent idea. However, one must determine why those genetic disorders exist in the first place. If it is caused by the environment and diet. These two pieces of life are common among all people "where you live" and how you eat. Once the cause is determined those people should also look to avoid what is causing the genetic disorder in the first place.
Gene therapy is a robust set of practices that can be part of a healthy society. But they are expensive therapies, so, as a culture, we must also make totally sure that everyone has access to this type of health or no one should have access to these potentially life-increasing technologies.