Given the LCME (accrediting body for medical schools) focus on racial/ethnic diversity, every school is going to at least make an effort to recruit a racially and ethnically diverse student body. Diversity of socio-economic status in childhood is not a criteria AFAIK for accreditation. That said, there is some thought that the people most likely to locate their medical practices in low income settings, either inner-city or rural, will be people who grew up in such settings and are familiar with the culture and lifestyle. Ergo, it behooves us to train such people to be physicians. Furthermore, schools are putting more emphasis on "grit" and if you can show you finished the race at the same time as someone else (for this metaphor, let's say you achieved 3.9/512) but you started 200 meters behind the other competitors in this 1500 m run (e.g. coming out of a poor performing public HS, and with neither parent having attended college), then you are someone who certainly has what it takes and is perhaps stronger than other applicants who put up the same numbers.