I think that for most med schools, grades are pretty much at the very bottom of list in regards to importance. They will use low grades as a sifting tool to through handfuls of applicants out, but MCAT, essays, interviews, experience are all much more important than GPA.
I think it's different in the United States. In Canada, performance on the MCAT isn't emphasized as much, especially because many schools don't require it (some because they're French, others because meh). Also, our GPA cutoffs tend to be higher. The words of wisdom that I've picked up is to focus focus focus on your GPA which should be your priority because it gets your foot through the door, ie above 85% of all other applicants (the interview stage takes about 50% of the remainder).
Jokes aside, to answer the OP, depending on where you go grades will be used to judge one's potential success in a career.