There now follows some alphabet soup...
ESC therapy uses (or rather, will use, only done in the lab so far) SCNT - Somatic Cellular Nuclear Transfer to create new brain or heart or muscle or whatever cells for people who don't have enough.
The technique is to take a cell from a blastocyst, that's the earliest, youngest form of embryo, just a ball of undifferentiated cells, and squeeze out the bit with the DNA, the nucleus, and replace it with a nucleus taken from the patient, for example from a skin cell, of which the patient has lots and can easily spare a few.
The blastocyst cell is then replicated by clonal expansion, ie it is persuaded to mulitply into lots of cells, which can then be inserted into just about any organ in the patient where they convert into the right kind of cell for the organ.
Who will benefit? Well, everyone loses brain cells though-out their lives - we could all use a few more when we get old. Suffers of Parkinson's disease have big holes in their brains where there used to be brain cells. They could do with a few more cells. Anyone who has suffered a "cardiac event" will have reduced cardiac function which could be addressed by inserting new heart cells.
So where do we get our blastocysts from? That's the problem. They have to come from embryos which never made it as far as becoming a baby. This doesn't involve abortions though; in the US alone there are tens of thousands of blastocysts sitting in freezers, a surplus from IVF production. In the normal course of events these blastocysts would degrade over time and eventually get thrown out. They have no chance of ever becoming a person.
A Christian might take the view that the blastocyst already has a soul. But here there's a problem. How many souls does it have? A blastocyst could be implanted in a woman and become a baby, or it could divide and become twins or triplets! The "soul" argument is fraught, and should not be allowed to stop ESC research.