It might be easier to answer this question if some of the diseases stem cells are targeting is listed.
The Promise of Stem Cells
Studying stem cells will help us understand how they transform into the dazzling array of specialized cells that make us what we are. Some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur somewhere in this process. A better understanding of normal cell development will allow us to understand and perhaps correct the errors that cause these medical conditions.
Another potential application of stem cells is making cells and tissues for medical therapies. Today, donated organs and tissues are often used to replace those that are diseased or destroyed. Unfortunately, the number of people needing a transplant far exceeds the number of organs available for transplantation. Pluripotent stem cells offer the possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases, conditions, and disabilities including Parkinson's and Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis.
http://stemcells.nih.gov/info/health.asp. What are the potential uses of human stem cells?
Stem cell research contributes to a fundamental understanding of how organisms develop and grow, and how tissues are maintained throughout adult life. This is knowledge that is required to work out what goes wrong during disease and injury and ultimately how these conditions might be treated. The development of a range of human tissue-specific and embryonic stem cell lines will provide researchers with the tools to model disease, test drugs and develop increasingly effective therapies.
Replacing diseased cells with healthy cells, a process called cell therapy, is a promising use of stem cells in the treatment of disease; this is similar to organ transplantation only the treatment consists of transplanting cells instead of organs. Currently, researchers are investigating the use of adult, fetal and embryonic stem cells as a resource for various, specialized cell types, such as nerve cells, muscle cells, blood cells and skin cells that can be used to treat various diseases.
In theory, any condition in which there is tissue degeneration can be a potential candidate for stem cell therapies, including Parkinson's disease, spinal cord injury, stroke, burns, heart disease, Type 1 diabetes, osteoarthritis, rheumatoid arthritis, muscular dystrophies and liver diseases.
In addition, retinal regeneration with stem cells isolated from the eyes can lead to a possible cure for damaged or diseased eyes and may one day help reverse blindness. Bone marrow transplantation (transfers blood stem cells) is a well-established treatment for blood cancers and other blood disorders.
9. Are stem cells currently used in therapies today?
Hematopoietic stem cells (HSCs) or blood stem cells, present in the bone marrow are the precursors to all blood cells. Blood stem cells are currently the only type of stem cells commonly used for therapy. Doctors have been transferring blood stem cells by bone marrow transplant for more than 40 years. Advanced techniques for collecting or "harvesting" HSCs are now used to treat leukemia, lymphoma and several inherited blood disorders. Cord blood, like bone marrow, is stored as a source of HSCs and is being used experimentally as an alternative to bone marrow in transplantation.
New clinical applications for stem cells are currently being tested therapeutically for the treatment of musculoskeletal abnormalities, cardiac disease, liver disease, autoimmune and metabolic disorders (amyloidosis), chronic inflammatory diseases (lupus) and other advanced cancers. However, these new therapies have been offered only to a very limited number of patients.
10. Why is cord blood a valuable resource?
Cord blood is rich in hematopoietic or blood stem cells and is currently being used as an experimental alternative to bone marrow transplantation. The collection process is completely non-invasive, the host-donor match required for transplantation is less stringent and cord blood has fewer mature immune cells and thus poses a lower risk of graft vs. host disease.
For more details on cord blood collection, storage and current therapies:
http://www.nationalcordbloodprogram.org
Secondly, the resulting embryonic stem cells could be developed into a needed cell type, and if transplanted into the original donor, would be recognized as 'self', thereby avoiding the problems of rejection and immunosuppression that occur with transplants from unrelated donors.
http://www.isscr.org/science/faq.htm#7