Is it though? I might be missing something but it's not obvious to me that it should disproportionately affect anyone. I wont reiterate the points about Avg number of apps discussed early on in this thread, but the main reason applicants feel they need to apply to an ever increasing number of programs is to increase their chances of receiving enough interviews to feel confident in their ability to match their preferred specialty and the competition for interviews is itself driven by the number of apps received / interview slot at any given residency program.
More importantly, from a big-picture view it doesnt seem to me that you can leave the problem of app proliferation untouched. Switching to quintiles would still give applicants from any medical school an opportunity to differentiate themselves and therefore understand their level of competitiveness for X specialty or Y program within X. But if you leave app proliferation as is then, in some very competitive or small specialties (like Derm), you would make it much harder for PDs to make meaningful decisions about who to interview or not -- e.g. if every single Q1 applicant is applying to every single program in the country regardless of whether or not they are likely to actually rank the program highly if interviewed, you stall the disbursement of interviews to candidates most likely to accept/need them and therefore increase the probability that otherwise desirable applicants may fall through the cracks. Who is most likely to fall through the cracks? Precisely the types of students you and many others are concerned about.