"It might be better to ask candidates to code something up prior to coming in and then go over the code together in the interview." That is entirely reasonable

"It might be better to ask candidates to code something up prior to coming in and then go over the code together in the interview." That is entirely reasonable and a lot of employers do this. So why bother with the ones who want you to demonstrate whiteboard chops that don't have any relevance to the daily work you'd be doing? Remember... the interview isn't just for the employer. It's your only opportunity to determine if you're interested in working for the company. If the interview is a one way street and doesn't give you the opportunity to learn anything meaningful about the work you're really going to be doing, how would you feel good about joining the company even if you ace their inane whiteboarding exercises?