Good point, I find it kind of unnatural to transform a developer community into a recruiting platform. I think it's just companies searching for a solution to solve the resume problem.
It's also an incentive for developers to earn karma as it may reflects on a career opportunity. Other popular developer websites, such as github, also have a way to differentiate /popular/ or strong developers (number of stars/fork). Personally, I prefer the github as a developer and a manager mostly because I'd feel more comfortable working with an employee that crack lots of code rather than one who spend lots of time answering questions.
as a developer and a manager, i'd love a guy who could productively answer questions. it means that he can both solve problems and help other developers who might be struggling with the same issues.
this is the kind of developer who will make sure his code is commented from the perspective of letting someone else see how it works, and that your internal dev wiki is well gardened. if you ignore the value this sort of person can add, it is entirely your loss.
I think I'm with you on this one. Crappy code can still get a lot of stars/forks and great code can pretty much get ignored if it's not the flavor of the month with the majority bandwagon. I think Github is great if you want to look at someone's code, but being able to successfully answer questions, while more academic than utilitarian, in general identifies those with a deep and thorough understanding of the technology. SO gets gamed though.
Ultimately, I suppose that either are a good bet if you're looking for someone who stands out in a certain way. In both places you should vet the quality of the code or quality of the answers. And if you're the type of employer who uses these as metrics for rating candidates, then I think you should also probably give your employees time to work on their Github projects/SO profiles (I didn't intend the irony there, but it's there all the same).
Such a transformation was made with the designer and developer community Forrst and has been quite successful. I'm sure there are many more niche communities I'm not aware of -- to avoid the tragedy of the commons such quality communities are often not heavily advertised. For instance, Forrst is invite-only.