(1) The person can apparently code when he's allowed to pick what he wants to work on and doesn't have any real pressure, and
(2) He has time to work on open source.
Most companies need people who can code well under pressure and when they have to work on something they don't necessarily want to work on.
By screening by OSS contributions you will indeed eliminate one kind of false positive--they people who can't code at all. Other kinds of false positives, such as people who can't work well except on self-selected projects, will get in.
And from the developer perspective, just the fact that it helps you avoid idiotic CS101 tests over the phone at every interview is enough to make it worth.