When I look at the best programmers I know, none have them have done any open source work whatsoever. This group of people ranges from IBM fellows who created some of the earliest minicomputers to IMO medalists who are unbelievably fast problem solvers. I'm not going to say that's a generalizeable statement. It's not, and that's the point -- it's a side effect of spending a career working in teams where people care more about playing with their kids or doing research or whatever than open source.

It's not even that they don't work on projects in their spare time. They do, but they're not part of open source culture, where people share things by default. They're satisfied with having made the thing itself, and they're not going to put their hardware or software project on github any sooner than they're going to make a website for the house they built from scratch.

If you work at an open source shop and hire with this attitude, all you're going to do is make sure that you have a monoculture in this dimension. That's not the worst thing in the world, but it filters out people who don't particularly care about open source, which is the majority of programmers. If that's a moral imperative for you, I don't have any objection to that. Otherwise, I don't see why you'd want to do this.