I disagree completely about your theory. I have been on both ends of the whiteboard interview an almost uncountable time during my career. What it has always come down to in my mind is a collaborative process to solve a problem where the one performing the task is trying to solve the problem, while the giver of the task makes suggestions and points out weaknesses before they get out of control. I'm sure there are sadists who just want to watch people squirm while trying to solve an extremely complicated problem, but I don't think I've ever run into that in an interview and it has never been what I've put people through.