It is not a question of morality, it is a question of diversity. Without diversity the species will fail. This is a natural constant that all living creatures adhere to. If we highlight certain traits through genetic modification we are excluding other traits thus limiting diversity. If we eliminate genetic diseases we could inadvertently create new ones. In the short term these are not issues that a parent who wants to remove the predisposition for something like leukemia would worry about, however over the long term who knows what it could lead to.
Also, as it stands now the science is leading not toward modification of embryos but instead of selective fertilization of certain embryos. This is a DISASTROUS process if it becomes a normal occurrence throughout humanity.