I think it is excellent advice. Every job opportunity of note, including a recruitment letter from Google (and, probably, my company participating in Y Combinat

I think it is excellent advice. Every job opportunity of note, including a recruitment letter from Google (and, probably, my company participating in Y Combinator) I have ever gotten came from my involvement in Open Source software. I also never hire anyone who doesn't have some Open Source software out there. Why would I? Open Source has so many great developers with good examples of their work in the wild. It's so easy to see who actually delivers and who just happens to be lucky enough to be on a good team at a company. Hell, I even choose designers based on their work with Open Source software (others do, too; my favorite designer, David Vignoni, stopped doing work for me many years ago when he went to work for Meebo...I'd still be sending him money for new icons today, if he'd take it).