Of course, you can't fully describe a person by just a number. It's theoretically possible that a highly reputed Stack Overflow user could be an awful programmer at work, but I'm sure that the intersection of "highly reputed SO users" and "people you'd regret to hire for a programmer position" is a very tiny set.
I think that number contains much more significant information for that kind of position than whatever you could extract from a CV or an interview.
Sure it can, if I think you are more invested in your badge-earning game mechanics than in the money-earning game-mechanics for which I pay you.
Of course, the chances that I review your SO score with this mindset seem relatively low, since in order to cover this possibility, I also need to check out your GamerTag, gear score, and Wikipedia edit history.