False dichotomy. I'm already earning money doing real work. I'm not applying so that you can cut into my family time. I'm applying because I want work that's more interesting or more money or who knows what. If you really want to know, ask, instead of assuming ;)
EDIT: Clarifying the "more interesting work" expression.
Apart from the money, such a "audition project" could help you a great deal in deciding whether this company actually meets your expectations.
You're of course right in that it will be difficult to do it like that for someone who already has a fulltime job.
I think the audition project is a good idea for people who don't have restrictions like the above, but should not be used to reject those for whom it doesn't make sense.
But if it's short and interesting, it usually falls into "I don't care" and stays there.
Note that he proposes the audition project to be "a regular consulting gig with an hourly rate", spent on-site doing real work.
You don't have time to earn money doing real work? Then why are you applying?
And even for a "truly good" professional who has his pick of a dozen offers, I could see this as a good idea, since they have just as much reason to be interested in the company's "real" work environment than the company has to be interested in their "real" work performance.