There's no way--at the "top" schools, at least--that the interview is tremendously more important than everything else. Among all the very smart and qualified applicants interviewing at these schools, it is the rare applicant who cannot be socially competent and generally amiable for a short period of time. And because everyone realizes that most of these people can put on a 30-60 minute charade if necessary, the interview is, for the most part, merely one component--or a sub-score at some schools--that is used in the admissions process. Consider the following...
When I was interviewing at HMS, my student host told me about his interview at PENN, where he interviewed with a physician who was mentored by his parents when they were physicians in the PENN hospital system. When my host walked into the room, his interviewer said, "Wow, you must be the son of so and so. Your parents were two of my most influential mentors. It is really a privilege to meet you." My host was like, "Yeah, sweet, nice to meet you too!" After shooting the **** for nearly an hour, the interviewer said, "Don't worry about anything. I am going to write you a fantastic letter." He was not accepted
. If everyone were equal at the interview stage, which is an intrinsically ridiculous assumption, my host would have undoubtedly been accepted. (Though I will concede that this story is anecdotal, it is no more so than the "My friend with mediocre stats was accepted at SOM X after an awesome interview, so the interview is obviously the most important element in the admissions process" garbage you have no doubt been reading already.)
While the importance of the interview is admittedly dependent on the school, it is also dependent on the reputation and importance of the person interviewing you (e.g., Dean of Admissions vs. Dr. Altruism who serves on the Ad Com every third year by default and relishes the prospect of biting the heads off of any Joe Benchtops and Betty Beakers that have the misfortune of interviewing with him). So even at the same school, all interviews are not equal, which makes quantifying their importance an exercise in futility. I wouldn't bother. Just make sure you don't bomb it!