I valued open source contributions highly when I was interviewing candidates: it demonstrates that you're someone who is passionate about programming and enjoys building things in your free time. That's exactly the sort of candidate that I think a lot of startups ought to be looking to hire.
But I think the question of open source contribution is completely different from "show us some code." The latter is intended to get a look at some of the actual work you've produced; the former is just another part of your work experience, formal or informal.
That's exactly my point. No one here (at least in my experience) focuses on open source contributions per se. The end goal generally is to look at some code that a candidate have written, not necessarily the open source one.
Of course there are exceptions. For certain positions you will get a big bonus for any contributions to a specific high-profile open source project. But this has less to do with a code you wrote and more with domain knowledge you picked up along the way.