My daughter ('13) does alumni admissions interviews for Dartmouth, and this year she is thrilled that for the first time one of her interviewees got accepted. Now the goal is to convince her to enroll.
My other daughter (Harvard '08) gave up interviewing for H a couple of years ago. It was just too discouraging that year after year none of her interviewees ever got in no matter how wonderful they were or how highly she recommended them.
My D is an alumni interviewer also and interviews people in her professional life. It is funny the number of people who have googled her in her professional life that want to tell her about their connection to Dartmouth 
Wow, this sheds a new perspective. I had previously thought that alumni interviewers only gave phenomenal recommendations for 1 or 2 candidates, and didn’t say much for the rest, and that’s how adcom chooses people. But IN guess your other daughter (Harvard) gave great recommendations for A LOT of the candidates? And still, very few got in?
I am wondering, from the perspective of your daughter, does she feel she has any impact on the decision process when she’s writing these evaluations?
My impression is that getting a poor recommendation from the alumni interviewer might hurt your chances for acceptance, but getting a glowing recommendation won’t help your chances all that much. There are just too many great kids applying. Getting a great recommendation from the interviewer is like having great grades or high SAT scores - you and about 20 - 30 thousand other applicants have those too.