Nope, we have a 100% cash back guarantee to the employers if the recruit doesn't work out (not just a credit for future services, like most headhunters/agencies).
The incentive? Having integrity perhaps? Besides, it's not even financially worthwhile to job hop just to get a bonus. The bonus works out to be less than eight days of salary. If you're really a top developer, you're not going to tarnish your reputation for such a paltry sum.
I wish "having integrity" was a more universal incentive, but you have to see it from all angles. I can see how top talent would avoid it, but what about the others?
At least from what I have seen, the Valley is much friendlier when it comes to job-hopping than most other markets. Maybe changing jobs every year is a little extreme, but certainly every 2-3 years is not out of the question.
You're talking about something different. The comment I replied to asked what the incentive was to prevent the employee leaving soon after starting the job, such that implementing a policy of only paying out the bonus after 3 - 6 months would curtail such an issue.
They should make a requirement that you are at the job for 3-6 months before you get the pay.