there are 2 ways to look at interviews.
if your stats are superb and your overall application is above the standards of teh school you apply to, an interview is used to confirm their admission. if you speak and write well at the interview, you're guaranteed acceptance.
however, if the numbers are great, but your personal skills are lacking, they can reject you. but nonetheless, if your app is good, the interview is used as an insurance.
HOWEVER, if your app is average or even below average, its a way for you to SELL yourself. you should be spectacular at the interview. you have to use your interview to overshadow your average stats or ECs.
so there's 2 ways to look at it.
if i was to interview a 3.8 GPA applicant, if he interviews good (not great), he'll still get accepted.
if i was to interview a 3.4 GPA applicant, if he interviews GREAT, he'll get accepted. if not, i'll take the guy w/ the higher GPA.
overall, i thikn most of the acceptances are sent out to most of the interviewees w/ great numbers. a few will go to the borderline who shine on interview day.