Candidate 1: "I brought this fanfold printout of the source code for _____ I wrote with me, have a look.
Candidate 2: "I can't show you any of the code, but trust me, it's good."
Github merely drags that conversation forward from 1981 up to 2011. When you and I choose to write code that is locked away in somebody else's vault, we ought to charge extra to compensate for the fact that we might as well have been surfing Oahu.
The argument that this isn't fair to many programmers has also been going on at least for the last 30 years. Fair or not, it helps a lot in convincing an employer you're worth hiring.
I like that idea. Part of the value from working on open source projects for companies is the public nature of your contributions: It's easy for you to take your commits to Chromium and show it to Facebook or Apple when looking for a job there.
Companies may not like that idea, however.
Perhaps, but it's a lot less than the cost of not having a probationary period and winding up stuck with a poor member of staff indefinitely because you couldn't do the impossible and spot every problem candidate during a momentary interview.