What he is calling "spec work" would be better described as a "challenge project". I think these provide a lot of insight that you wouldn't have the opportunity to receive via the standard portfolio/github and interview process. If you give a job candidate a small project (3-5 hours tops), you're able to see how they solve problems and most importantly, how they interact with your team.

I've asked "A player" candidates to perform challenges, and I have happily performed them myself to get a job. And as a candidate, there have been a couple of time where I've realized via the challenge process, that I wouldn't want to work with a particular group.

It's a smart use of time by both parties to quickly determine if the candidate is indeed a good fit for the position.

Spec work, as in completing a project to receive consulting work, is an entirely different debate.