Back when writing software was a hobby I effortlessly aced whiteboard interviews. As I came to identify more and more with my work I started to freeze up under the scrutiny and to resent some jerk half my age and with a quarter my experience daring me to impress him, as he sniffed and fidgeted and made misleading suggestions. Then, having failed the interview, I would come up with three separate approaches to the problem in my head as I walked back to my car.

I only freelance but were I onsite I would refuse these things, since the interviewer has no training anyway I would just take over the interview.