If you get rejected post-interview, more than likely there was something in your interview that got you rejected. Of course it varies by school and on an individual basis, but in the general scheme of things, if you were invited for an interview, your application is good enough to potentially be accepted. Contrary to what people in this thread apparently think, they don't invite people who have no shot. In that sense, I do see benefits. Benefit to the school for not accepting someone who wouldn't fit there. Also, depending on what the rejection was due to, I can even see there being a benefit to the student. Yes, I know, it's still better to be accepted than not accepted, but if you're going to be miserable at a school because you don't fit, maybe it's better to be rejected than to hate your life for 4 years like I know some do.
But you're right, the interview is as much you "interviewing" the school as the school interviewing you. I got pretty bad vibes from 2 or 3 of the schools I interviewed at, and I wouldn't have known that if there was just a phone interview or if there was no interview at all.
Y'all need to quit your bellyaching 🙄