Scientists continue to tell reporters that the genetic diversity of existing stem-cell lines is more important than the mere number of stem-cell colonies. Why do scientists want genetically diverse stem cells?
Stem-cell research is so new that scientists aren't certain what stem cells can and can't do. Some stem cells could be endowed with genes that make them easier to manipulate, or more amenable to being directed into a specific type of human tissue, or less likely to "crash" and die, for example. Unless scientists have access to cell lines that reflect the genetic variety within the human race, they fear their understanding of stem cells will be limited.
Down the road, if stem-cell research lives up to its promise, genetic diversity could become even more important. Scientists hope to grow specialized cells that could, for example, restore liver or heart function in sick people. If that comes to fruition, the available stem-cell lines would have to reflect genetically the variety of the human species in order to provide treatments for everyone and not just a limited genetic slice of the planet. But scientists are nowhere near providing those kinds of treatments.
Note: Genetic diversity is not the same as ethnic diversity. The genetic differences between two random individuals from a particular ethnic group would be roughly the same as the genetic differences between two individuals randomly selected from different ethnic groups.