[gaius]

What if you consider part of someone's job to be communicating concepts to people, possibly with the help of visual aids and diagrams?

This is incredibly common, and you tell the candidate in advance that they will be expected to give an n-minute presentation on a subject of their choosing (need not even be tech) so they can prepare.

Tell me this: when did a mob last ambush you at your desk and demand that you invert a red-black tree on a whiteboard that they happen to have with them right now? I'm guessing that never happens where you work. If it does then fair enough :-)