The embryo's moral worth is not determined by its instrumental value for others. This would be to absolutize utility and to devalue the lives of all other classes of vulnerable human beings. The societal consequences are grave indeed.
These ethical objections cannot be over-ridden by the claim that the embryo is entitled to a "special respect" but that this respect can be violated if there is sufficient benefit for others. Respect is inherent in the moral status of what the human embryo is in fact. Respect is neither conferred nor removed by arbitrary social convention or convenience.
Nor can the ethical issues be side-stepped by calling the blastocyst a "pre-embryo." This is a euphemism of convenience with no ethical or biological justification. There is no arbitrary point at which we can logically confer or withdraw the moral claim of the embryo for protection of its life.
Couldn't have said it better myself.