In the case you describe, you've already done the more "valuable" dev work of thoughtful analysis/design, and you are merely presenting that work.
That's far different from being forced to devise that design at the whiteboard, singlehandedly, with onlookers.
That's not to say that devs don't hash out designs collaboratively via whiteboard (yet a different application). That certainly happens and is useful. But, you're likely to find that--even in those situations--team members have considered the problem offline prior to the collaboration.