tl;dr; - Too many good people have 'portfolios' that they can't legally show me; until that changes, I have to ask people to code on a whiteboard.

Here's the more nuanced version (in which I largely agree with jnbiche):

The stuff listed in that article under "An Alternative" describes the exact tech interview I used to give. And we had several people pass it with flying colors who we later had to let go because they couldn't actually translate a requirement into code.

So we adjusted our interview to make sure that people had the ability to write basic code. I'm not talking about stupid puzzles or API quizzes. I'm talking about 'show me you understand what recursion is'. I'm talking about 'given a detailed description of the FizzBuzz problem, can you write me a loop (in any language; pseudocode is fine, too) that outputs the right answer? And can you adjust your code if I change the requirements slightly?' Stuff that anyone who codes on a daily basis should be able to breeze through.

This was dead-easy stuff. Just me and the candidate in a room - there wasn't a giant audience, there was not ticking clock, and I was explaining the algorithms. And again, quite a few people could not do it.

If they could do it, we moved on to the original interview where I asked about technology, past projects, etc. Some people didn't pass that part of the interview.

Believe me, we were stoked when we got a candidate who listed open-source projects on a resume or brought it up in the interview, because we got to see real-world code that the candidate had written. But that was a relatively rare occurrence.

This brings me to the crux of the 'until other, better signals of ability are widely available' comment. I think it's widely agreed (at least, in a place like HN) that code is the best signal of ability. But there are many, many candidates who write excellent code that no one outside of their company will (or even legally can) ever see. If I turned away candidates just because they didn't have a github account, I would have missed out on some of the greatest coders I ever worked with. There are plenty of people who passionately write excellent software every day, but who do so under NDA. They may not work on OS projects in their off time because they may not have any (or they have kids, or other time-consuming hobbies).

So for a lot of fantastic coders, the best possible signal of their ability is not publicly available. I would even go as far as to suggest that this may be true for the majority of fantastic coders. Everyone has to pay the bills, which means 8 hours of writing custom business software for a lot of people. I know for a fact that some of the best stuff I've ever written is buried with companies that failed, never to see the light of day. I can't show that in an interview. How many other people are in the same boat? http://xkcd.com/664/

Obviously, one huge part of the solution to this problem is for companies to open-source their code. That movement has already begun; lots of successful companies have come to the realization that keeping their code 'secret' has no value while allowing outside review and contribution has immense value. And some smart companies which use a lot of open-source tools and libraries are paying their employees to contribute bug fixes and features. Those trends are starting to address the 'widely available' part of my comment, and the day may eventually come where employers can simply throw away resumes which don't have any links to code contributions. In fact, if you're looking for another bullet point on your presentation to your employer about "why we should do open source", I would suggest mentioning the hiring benefits to the entire community.

But until we hit that critical mass, I'm still going to have to ask people to implement n-factorial on a whiteboard.