Of course P=MD, but if you want a say in what KIND of doc you want to be grades matter. I would think the majority of people would want to keep their options open.
(Source:
http://www.nrmp.org/data/chartingoutcomes2009v3.pdf)
Not generally true. Competitiveness on SDN is vastly overstated. As long as you're a US senior (MD school), with about a 220 USMLE (The mean for US seniors is 224.3, s.d. = 19.6), and you rank >15 programs, your competitiveness in matching highly competitive specialties is very high, and nearly 100% for matching in general. You may not get your first choice of location, but you'll likely get in. Many highly competitive specialties have plenty of positions for US applicants, and they are heavily favored over international/independent applicants.
Preclinical grades are highly
insignificant (many schools don't even have them). But if you can ace those exams then you'll probably ace the boards, too. Grades "matter" only if that means you know the material better.
Success is contingent on adequate/good board scores, good grades in the clinical rotation of the specialty you're applying to, gleaming letters of recommendation from relevant faculty on that rotation, and (most important) applying to as many programs as you can. Inflexibilityis your real enemy. This is what the residency directors actually tell us in med school, and it's backed up by the facts.
Understand that most medical students want nothing to do with your personal choice of specialty. Even if you're gunning for a ROAD specialty, you would be mistaken to assume that top students are all applying. Many would rather be cardiologists, transplant surgeons, neonatologists, etc. And by the way, that match list is going to read: internal medicine, general surgery, pediatrics. Yet another reason why match lists are useless (yet still heavily played on SDN) - you never know what fellowship lies down the road.
EDIT:
Readers, remember to stratify population groups when you're looking at data. Otherwise, the lack of success of independent applicants skews the numbers. If you're a MD student at a US school, and you're about average, then you're doing just fine. If you're in a different educational situation, just be aware of the hurdles, and of course DO students have their own match as well.