Aside from the well-documented finding that adult stem cells are yielding better results than embryonic stem cells, the short answer to your question is this: Embryonic stem cell research promotes abortion and abortion rights is an all-important issue for the left.
Here's an article from April 2002 on results we knew about, even then, from adult stem cells, including the treatment of Parkinson's and spinal cord damage (and which is still being ignored in favor of ever tiny report on embryonic stem cells):
http://www.nationalreview.com/comment/comment-smith042302.asp
You are exactly right,twhitak. There has to be justification for the destruction of all embryos in order to continue justifying the emptying of uteruses.
Also, the purpose is to learn from the lab animals, uh, human embryos, uh, spare embryos that are going to be wasted, anyway. So that when human cloning is perfected, clones can be produced, implanted in a woman or some animal that has been modified to carry a human fetus to near-term when the adult stem cells can be harvested. That sounds so complicated and evil, but that's what I see coming.