I don't think SO Rep is really a good candidate for "the one number to rule them all" when it comes to hiring. As much as we'd like it if everyone used SO, they don't. A great majority of the users of SO never gain enough reputation to distinguish them from each other based solely on rep. You can look at the reputation distribution for SO at http://stackexchange.com/leagues/1/week/stackoverflow in the sidebar under Total Reputation. Even if we say that having 200+ rep is the signal, that's only 41K people in the world.

So if someone decides on using SO Reputation alone to qualify candidates, they can't effectively solve the primary problem of sourcing those candidates.