With regard to #1, your chances of working your way up to getting code review comments from Linus Torvalds or Rob Pike is a lot better if you're contributing to (other people's!) open-source projects than in almost any non-OSS work environment.
I don't think people have this attitude because of these theoretical reasons, though. I think they believe it for empirical reasons — because they don't know any first-class programmers who only code at work and don't work on open-source software. Do you? I was just trying to explain the evidence I've observed, not trying to convince you from first principles.
But, doesn't it follow that you wouldn't know of these great programmers precisely because of the fact that they only code at work. You wouldn't have visibility of them unless you worked with them... and I suspect that for most of us the number of other programmers we work directly with over our careers is somewhere around 100 - 200 (Just a guess based on how many I work with directly and how many times I'm likely to change employers)... That's a very small slice of all of the programmers...
Though it occurs to me that participating in OSS would allow you to work with far more developers than you would otherwise.Though I'd argue that it isn't really as direct contact as the workplace.
Also, in retrospect I targeted "OSS" a lot but what I'm really talking about is any side project that isn't your day job. I don't want anyone to get the idea I'm attack OSS.
With regard to #1, your chances of working your way up to getting code review comments from Linus Torvalds or Rob Pike is a lot better if you're contributing to (other people's!) open-source projects than in almost any non-OSS work environment.
I don't think people have this attitude because of these theoretical reasons, though. I think they believe it for empirical reasons — because they don't know any first-class programmers who only code at work and don't work on open-source software. Do you? I was just trying to explain the evidence I've observed, not trying to convince you from first principles.