The thing is, the motivation is different when it's fake internet points than when it's actual money. The relationship between the people is different, too. The way it works now, the relationship is a cordial, friendly, familiar relationship (even though the people don't actually know each other in many cases). Throw money into the deal, even the paltry $10, and now it becomes a business relationship, with all that entails.
Maybe. I think if a site like GitHub or Stack Overflow added bounties they'd do it in a way that kept the spectrum more on the relationship side, rather than the employee/employer. And the bounty is an extra perk.
Stackoverflow does have a bounty system . you can only pay with your fake points though , allowing ppl to pay with real cash is ripe for abuse.Their points economy is very carefully built, just like any successful in game economy, hyperinflation is major problem is a problem all of them have to worry about.