I'm convinced that they are. Sure, it's not perfect, but the set of qualities that impress me in a whiteboard are generally the same qualities that I look for in an engineer. Can you communicate well? How do you approach problems you've never seen before? Can you write readable code without thinking too hard about it?

I'm not really selecting for people who are really, really good with binary tries -- I'm selecting for people who display those qualities, the question is pretty much incidental.