My biggest problem with articles like that is their inability to take individuals' experiences into account. Like @Joe said,
I don't expect to see a blog post saying "Activities that are fun and beneficial to you should be limited to only X hours a week."
For me, the work-life balance has always been about fulfillment. [[http://en.wikipedia.org/wiki/Self-actualization|Here's]] a good start on that idea. I don't know when the shift happened, but my father's generation had a job and had outside hobies. Think of Dad going ot the office every day, but doing woodworking or car repairs at night. I've noticed in recent years that instead of having "day jobs" a lot more people are figuring out how to parlay what they love into an income stream. Those people, myself included don't face the drudgery of work because they love the work so much. Their work is fulfilling and fun and energizing.
I agree with @Joe that the study is likely based on 40 hours of doing something that doesn't fulfill you.
I believe that finding that fulfillment should be paramount. If that means stopping work after 40 hours to pursue your passions, by all means do that. However, if you're fortunate enough to make money from your passion, the 40 hour rule ceases to be relevant--within the context of maintaining healthy relationships around you, of course.