This has been discussed in some of the other threads, but I will chime in again. One of the main reasons AMCAS or Adcoms consider racial background (apart from statistical purposes) is to improve health of minority populations. The goal is to bring up the number of physicians to (at least) match the percentage of minorities in general population. This is of course based on multiple studies indicating that minority members tend to practice in their communities. Using socio economical factors (which is somewhat in play if you fill out the disadvantaged section of your AMCAS application) will probably diversify medical schools, but won't produce the sort of "functional diversity" adcoms seem to be looking for. When racial information is used in medical admissions, the goal is to increase the number of minority physicians, not the number of poor people going into medicine (although the disadvantaged section can be used to do this). I think they make this point quite obvious.
Again, you should go to the other threads because all the questions you raised have been discussed many many times.