I would first get myself and my "mate" genetically tested to see what risks our child would face. Then I would only want to prevent genetically inherited diseases or disorders that could bring down the quality of my child's life or worse, end it. If you look up inheritable diseases and disorders like Tay Sach's, Edward's syndrome, cystic fibrosis, schizophrenia, or agoraphobia, for example (there are tons of others), you can understand why a couple that could pass those along would want to ensure that their baby cannot get them. It's just an act of love and mercy.
But I personally don't believe genetic modification should be used to change physical or personality traits. Biologically, a child is a couple's offspring because they got their unique combination of genes from them. If a third party also creates a particular gene combination, then I feel that the child has, in essence, a third parent (science, I suppose, or whoever was responsible for altering the genes). That just rubs me the wrong way. I don't know why though, so I can't say I'm right in feeling that way. It's just how I feel, perhaps instinctively or morally. I wouldn't want it unless it could save my child.