The gene pool is nature's great experiment. Sometimes it is a great success, and sometimes it goes awry. When it does not work out well, those whose genes are somehow problematic are less likely to pass on those genes to a new generation. So, when we tamper with those genes, we might very well be creating more problems for the future than we solve. That is one reason to not tamper with genes. Another is that genes tend to be "bundled." There is some speculation that in the bundling, there are good traits and bad, so for example, it is possible that the genes passed down that carry a genetic disease might also confer some advantage. Thus, we run the risk of doing away with the good as well as the bad. That is another reason to be wary. Additionally, variability in genes is quite important because in a population with too much similarity, recessive genes emerge, often with very undesirable consequences. The more variability there is, as I understand it, the less likely it is to pass on many genetic diseases and conditions. Since tampering at will with genes implies that people will want to all converge on some "ideal" that includes intelligence, beauty, and strength, we could very well lose a great deal of variability.
Now, having said that, do I think that if I had a child who could be "fixed" through gene tampering I would turn it down? Not necessarily, of course. However, I do think that scientists, doctors, and people in general should at least consider the consequences, not just in the individual case, but also for the human race. So many "advances" have unintended consequences, from the introduction of rabbits to Australia to the giving of too much oxygen to premature infants, creating a generation of blind children.