And I'm not saying you shouldn't prepare for interviews. I'm just saying that companies that evaluate a candidate based on how they "perform" in an interview vs. trying to really get to the root of how they would perform in a job are doing themselves a disservice, and may be losing some good candidates because of it.
And if this is happening, that company should change their interviewing strategy to focus more on what's important, and get rid of anything that may get in the way of evaluating that.
Take your point on writing code on a whiteboard vs. writing on a notepad, just for example. To your point, it may be unnatural to code on a whiteboard. But, to me, it's also unnatural to code on a notepad. Actually, I hate handwriting altogether. I prefer typing a thousand times over. So in order for a company to really see how I code, they would hand me a laptop, or even better, allow me to use my own.
Of course, there are arguments to whether that would even be a truly accurate evaluation. My point is -- and again, this isn't necessarily in direct response to your post -- that good employers know how to get to the root of someones abilities, without letting these interview-specific strategies get in the way.
Edit: Disclaimer, I did not vote you down. :)