When I read the article, I gathered that there was a necessity for expert in A to make orals in B because their style of presentation. To achieve acceptance at B, there were non-relevant to A requirements.
Did I misunderstood?
The goal of qualifying exams to produce well-rounded students---or at least ensure that they are minimally aware of the major intellectual traditions in their field.
It's not crazy to expect that a soon-to-be physics professor understands how a battery works, at least roughly.
But you did not say "knowledge in A is not relevant to B" (itself an arguable claim, as my sibling commenter mattkrause points out (https://news.ycombinator.com/item?id=33910025 )); you seemed to be dismissing the validity of "knowledge in A" entirely ("There is always a professor of distinguished, but long forgotten and superseded past-expertise on the orals"). If you meant only the milder claim you make here, then, not being a physicist, I do not think I am qualified to judge, and so withdraw my objection.