They are precisely what human beings look like at that point of their lives. Freezing an embryo does not kill it, but merely arrests its development. This would eliminate the creation of additional embryos and their subsequent freezing, which is contrary to their human dignity and puts them at risk for subsequent research. Third, this form of research would allow embryos to be stripped of their cells and their integrity, reducing them from a subject to an object, from a human being with dignity to a source of organic material. Fourthly, the difficulties are compounded when human embryos are created solely for research.
Therapeutic cloning is an additional affront to human dignity.
Scientists have long recognized the principle that no experimental or research procedure should be conducted on human subjects if it provides no direct benefit or if the risks to the subject are inordinately great. In the case of human embryo experimentation, not only is there no direct benefit to the subject, but the embryo is killed. This cannot be done for whatever reason, even in view of the possibility it might provide advances in science and medicine that would benefit others. No amount of public benefit can ever justify the deliberate killing of a human being. The argument is particularly hollow when the same results could be achieved by alternate means such as the use of adult stem cells or the stem cells derived from umbilical cords or placentas.