It's one heuristic out of many, and it's a fast one. If you go through all 10-15 github profiles, and you find an awesome programmer, than you're done for that point in time. If you don't find any, you can use a more time-consuming heuristic.

Throwing out resumes with spelling mistakes is even faster, and if you don't find anyone, you can always go back. The same can be said for any filtering technique.

The problem here is that a resume with no spelling errors says nothing about a persons coding ability. The resumes without errors are lacking the negative indicator of poor spelling/formating. A strong Github profile, however, consists of many code samples and is a positive indicator of coding ability, making it a much more useful metric. This doesn't imply a knock against programmers with no Github profile, simple a leg up for those who do.