I understand bcantrill's point as a quest to remove uncertainty. He wants to interview people who meet certain basic criteria so he can focus on what makes one CS grad different from the other. If he interviews people with more diverse backgrounds he will have to deal with more diversity and the interview would consume more time and resources.

But an engineer or a physicist would probably be able to advise him on what he should sing his hard drives to make them run smoother. I don't think a compsi grad could do that ;-)