Sorry, how is that a contradiction? I'm limiting my scope to my anecdotal experience rather than making vast generalizations like you are. And the purpose of my analogy was to demonstrate the speciousness of your reasoning, not to make a direct comparison between medicine and finance.
But let me try to rephrase this. You said that it's not at all common for college students or medical students to smoke weed, but then backtracked and conceded that there are smokers, but just not at the top of your class. Let's take your premise to be true, that in a given class distribution, the top end contains relatively few marijuana smokers. What is the causation here? Without one, your correlation means very little. Do you assert that marijuana directly causes lower performance in otherwise successful students who have gained acceptance to medical school? If so, prove it.
I obviously contest this, but even if your premise were to be true, I say that it's more instructive to think about the opportunity cost of smoking weed (in that this could be time spent studying). Or, alternatively, one could argue that the personality of those who smoke weed doesn't lend itself to gunning for all As in preclinical results, which carry little clout in determining your residency placement.