That's a really big obstacle, but it's not insurmountable. My companies often ask if the candidate is willing to be a contractor for a couple of weeks. If they're not currently working, it gives them some income to continue the job search, and it allows us to see what it's like working with them.
I can usually tell that someone is unacceptably bad within a few days of working with them, so it makes sense to do those few days before hiring (if possible).
Imagine you have one position, and 15 people were screened for an interview. What would you do with 15 contractors? Even if you run a Microsoft department it would not make much sense.
Trivial problems are a good filter, as long as you manage to maintain a non-stressful interview environment. Whiteboard coding is a good thing, because it shows that one really understands things, not just memorised keystrokes.
Doing about 80% of my work on a piece of paper, I cannot really comprehend all the moaning about whiteboard being "detached" from anything practical.