In general, it would be pretty hard to evaluate a candidate just off their interview. Half the time the interviewer hasn't even read their personal statement or application and its very conversational not covering all your activities, etc.
They can attest that you were socially competent, interesting, were able to discuss your activities, or a great applicant based on what you said - and they can write in your comments that they think you would do well at the school.
But in the end they still have to take that and look at everything again.
Here at UCI after the interview they have meetings where they go through all the applications and the comments of the interviewer (interviewers are usually not present). It would be pretty confusing to just have the interviewer comments to look at (they are often very sparse).
From what I've heard from the admissions committees and what they used to tell us when we did student interviews: getting an interview means your numbers are decent and your application caught someones eye, the interview is to get rid of the real weirdos/jerks/socially inept students and occasionally if you're amazing you will end up with an advocate at the admissions meeting. For the vast majority of average students it doesn't help or hurt much. But I'm sure that varies by school.