My gut feeling is that when the marks are roughly the same, as they will usually be when the admissions committee has hit the interview stage, is that non-academic qualities then pre-dominate.
I might be a bit biased regarding this, but up here in Canada, a couple years back, this was the admissions criteria for UBC medical school, my school.
GPA: 40%
Interview: 50%
MCAT/Extra-curriculars: 10%
I suspect this ratio is skewed towards valuing grades more in the American medical schools. UBC, my school, usually only receives 600-700 applications for the 120 spots, and so has the resources to focus a great deal on the personal qualities of each student (interview counts for a whopping 50%). As most American schools are relatively easy to apply to via AMCAS, there are often over a thousand applicants per American medical school. You simply can't interview everyone, so you use a GPA/MCAT cut-off to weed out a large proportion of people.
Short answer: If your marks are the same as the humanities guy, then you'll both either make or fail the cut-off. You'll be on the same playing ground, and all the other factors of the admissions process (interviews, reference letters, travel experiences, luck, etc) come into play.
Ian, MS1
www.geocities.com/mdpremie