Sometimes it is of help. Even if you don't check the "disadvantaged" box, your parents' education and professions tell the adcom something about your household of origin. If one or both of your parents are physicians, then it might seem okay that you don't have any "shadowing" among your ECs - presumablly you have an idea of what doctors do based on conversations with your parents. If you are the first member of your family to attend college that tells a different story than if you are the offspring of a lawyer and a social scientist. There is some argument to be made for "diversity" within a class that focuses not only on race and sex but on diversity of experiences. At some schools, you will be the only applicant whose parents are ranchers or beekeepers. If one of your parents has died, it is an easy way to convey that information without going on about it at length in your PS.