Sure, but you could make that argument about anything you ask candidates in interviews. At some point you have to ask something that attempts to evaluate their

Sure, but you could make that argument about anything you ask candidates in interviews. At some point you have to ask something that attempts to evaluate their performance in an interview timespan. On the other hand, people's performance in whiteboard interviews may positively correlate with later performance as employees. In which case they let you make a reasonable assumption about later performance without being able to evaluate it directly, which is really what interviews are all about.