If a job applicant mentioned his SO (or SF I suppose) account, and you saw that they had a rep of X, what baseline of expertise would you associate with that?
Would there be a point at which their rep points are more indicative of their devotion to this particular site than programming knowledge?
Obviously, a diligent manager would dig in to find out how those points were earned before basing a hiring decision on SO rep points, and they are designed to measure community service rather than expertise, but the badges, etc. indicate that it can be used as a shorthand to establish a baseline of knowledge.