I find this point interesting, as to me, this would be an argument for, not against. Time constraints on homework assignments are a lot closer to the constraints you'd have programming for a job than "whiteboard coding" would be. I'd hazard a guess that doing a good job on an in-depth, open-ended problem given a few days to a week is a better indicator of ability to be a successful software engineer than being able to solve a logic puzzle on a whiteboard in fifteen minutes.