Dr. Robert P. George, joined by Dr. Alfonso Gómez-Lobo, of the President's Council on Bioethics wrote a statement as an appendix to Human Cloning and Human Dignity: An Ethical Inquiry (2002). In it, he argues with perfect logic and clarity that human embryos deserve full moral respect. It is the best argument I have encountered against human cloning, ESCR, et al.
The most important starting point in debating the moral respect owed to human embryos:
This point is based upon what the embryo is according to science, and not upon any religious beliefs. We MUST acknowledge that embryos are human beings before proceeding in any further debate.
A human embryo is a whole living member of the species homo sapiens in the earliest stage of his or her natural development. Unless denied a suitable environment, an embryonic human being will by directing its own integral organic functioning develop himself or herself to the next more mature developmental stage, i.e., the fetal stage. The embryonic, fetal, infant, child, and adolescent stages are stages in the development of a determinate and enduring entity – a human being – who comes into existence as a single cell organism and develops, if all goes well, into adulthood many years later.
...The combining of the chromosomes of the spermatozoon and of the oocyte generates what every authority in human embryology identifies as a new and distinct organism. Whether produced by fertilization or by SCNT or some other cloning technique, the human embryo possesses all of the genetic material needed to inform and organize its growth. Unless deprived of a suitable environment or prevented by accident or disease, the embryo is actively developing itself to full maturity. The direction of its growth is not extrinsically determined, but is in accord with the genetic information within it. The human embryo is, then, a whole (though immature) and distinct human organism – a human being. (emphasis added)
Since this is true, it immediately follows:
To deny that embryonic human beings deserve full respect, one must suppose that not every whole living human being is deserving of full respect. To do that, one must hold that those human beings who deserve full respect deserve it not in virtue of the kind of entity they are, but, rather, in virtue of some acquired characteristic that some human beings (or human beings at some stages) have and others do not, and which some human beings have in greater degree than others.Dr. George goes on in depth to argue why he believes this position to be untenable. You may disagree with his conclusion about the morality of using human embryos as mere instruments in scientific research (I think his argument is irrefutable), but you MUST admit that your position casts certain human beings into a sub-category undeserving of even life itself. In the words of Dr. George, "The proposition that all human beings are created equal would be relegated to the status of a superstition." There is then nothing in principle that would prevent casting other classes of human beings into the same category.
I am not willing to make this step in the name of science. If you are, please be honest about what is up for debate.