It's true that this sort of thing happens all the time and not just in the B-School process.
For instance, a few jobs that I barely qualified for interviewed me for a variety of reasons (alum of the school, family connection) even though I had a tiny, tiny chance of being hired when stacked up against the competition. I was even told once by an interviewer that I was only interviewed because my father in law would have made life miserable if I hadn't been. That was an unfortunate way to start off but it actually turned out to be a good interview even though it was obvious I wasn't getting the job.
I think for the most part your fate is preordained before the interview and (as has been said before) is just the final data point as long as you don't mess it up too badly or totally rock the interview. Interviews are so subjective I doubt that they significantly sway the adcom in most circumstances unless they are REALLY strong one way or the other since, like everything else, they are only a tiny fraction of your file.
I doubt anyone has ever talked their way into business school without the rest of their file being at least solid if not great...