I find the statement about legacies & URM to be surprising and somewhat absurd. This would be an example of "quotas". Unless the school is so fabulous that no one ever turns down an offer of admission (Godfather School of Medicine
"we'll make you an admission offer you can't refuse"), the school needs to make multiple offers in order to fill the class. I don't see how a school can cap URM offers to 3 and legacies to 8 (particularly if there are more than that number who are highly desirable applicants who would be a good fit).
Legacies don't seem to influence adcoms very much although I did hear a buzz about an applicant who would have been something like 5th generation (I can't remember if the applicant matriculated -- maybe he got a better offer elsehwere
)
From my experience disadvantaged applicants and legacy might get you a little bump (equivalent to an extra point on the MCAT and/or an extra 0.1 on the gpa) when deciding to grant an interview but it doesn't play much of a role at "offer time" except at the edge where there is just one offer left and two applicants who are by rank, in a virtual dead heat.