I would love, as an interviewer, to be able to do this style of interview every time.

It's astonishing how often you get nothing useful as an answer. Things like "my boss suggested it" as the reason for choices being made, or "not really" when asked about alternatives considered, or problems they ran into...

The hypothetical project whiteboard interview gives people who haven't done sufficiently interesting past projects and opportunity to show that they can think about them, at least...