I really don't have any idea whether it gave me an edge in the admissions process. It is a subtly way to identify yourself as a minority, which may be favorable for programs looking for diversity. Also, sometimes your McNair coordinators may have contacts in your department at other programs so perhaps this networking could be useful. I also recommend going to the national McNair conference.
At my school, they also offered a free Kaplan GRE prep course to McNair students, which would be great if you havent already taken the exam.