For those who view embryos as a collection of cells akin to other human tissue, research is acceptable and relatively uncontroversial. For many people, however, intuition tells them that the status of human embryos lies somewhere in between these two views. An embryo seems to represent some kind of life, with a different potential than either the sperm or egg from which it began only a few minutes before. And that potential strikes many of us as deserving of respect in how the embryo is used or treated. One potential supply of embryos for research is the many infertility clinics across the United States and around the world.
Frozen embryos hold limited promise.
Rather than search for matching embryos from the spare supply, it would be better to create embryos using sperm and egg from related individuals. Or for the perfect match, an embryo cloned from a patient's cells could yield implantable cells or solid organs without risk of rejection. For some, the ethics of the two cases are different. Using spare embryos that were created with the intention of trying to have a child but have become unnecessary for that purpose is somehow different from creating embryos with the intention of extracting their stem cells. Whether intention matters depends on your perspective.