I hear that interviewers are really interested in seeing how you think and communicate constantly. Yet in my experience, if you solve the problem exactly the way the interviewer is looking for in a reasonable amount of time, you pass 9/10 times. If you don't, you fail.
This is kind of the point, right? Most places I've interviewed are far more interested in your communication skills, logic and thought process than writing perfect code on a whiteboard.
Many of my friends have failed to see this is actually the reason they have you write code on a whiteboard.