NPR's The Diane Rehm Show today discussed the advantages of having a Hispanic woman (Sonia Sotomayor) on the Supreme Court. Because she'll be able to draw on a "wider palette" of experience, they say, she'll be a valuable asset and necessary resource.
Just because she had different experiences doesn't mean she has more, and certainly not ones more applicable to cases. Just because she'll be more empathic to women's issues or Hispanic culture doesn't mean she'll be more fair or that the court as a whole will be more fair. Yes, she can add something others can't, but in that endeavor aspects of a case can become more important than they should be. No net gain is obvious.
We are all merely the sum of our experiences so any fair judge would recognize that. This is what expert testimony is for. By expecting the justices to capture all that we are by their demographics alone, we hurt ourselves. Prioritizing gender or race (or sexual preference, or religion, or the presence of a disability, etc) requires us to narrow our field of search. It could well deny us someone far more accomplished, and fair, than whoever is left after we apply whatever parameters we happen to be prioritizing at the time.