" I understand that Harvard wants to give all volunteers a chance to interview, but it didn't seem like a useful exercise for any of the involved parties (me, the interviewer, or admissions officers)."
With something like 24,000 applicants, Harvard has a hard time finding alum volunteers who'll contribute the approximately minimum of 2 hours of time that each interview takes when one considers the time of the interview and the time it takes to produce a narrative report.
Trust me: The problem isn't that Harvard wants to give all volunteers a chance. The problem is finding enough alum to volunteer.
As for training: Harvard sends each interviewer a comprehensive handbook, but, understandably, it's not feasible to provide extensive training to those volunteers.
Consequently, students who interview should do whatever they can to learn beforehand interviewing techniques that will allow them ways of gracefully conveying the information that they wish to highlight about themselves.