As I've said in another reply, what if you write Ruby and Rails code, yet it's closed source or protected by an NDA? You could be a better programmer than most of the open source guys, yet no one would ever know.
Suppose you can buy one of two used cars. One is a nice Lexus and one is a nicer Ferrari. They both cost the same, but the Lexus has a detailed maintenance report and a clean Carfax. The Ferrari has no records (because it is under a mythical car NDA).
It would be perfectly logical to buy the Lexus, even though the Ferrari on average would a better car for the same money, as long as we assume non risk neutral actors. Information risk goes up, value goes down. Arguably, it's a great reason to contribute to open source, if you're a programmer, to reduce the information asymmetry in future hiring processes and thereby increase your perceived value.