Similar to my idea for marking answers:
This would incentivize the behavior we want to promote: voting on content helpfulness polls. This information helps community managers recognize the best and worst content. There are many use cases for this information, including featuring top content and correcting incorrect/unhelpful answers.
Solution:
If we want to promote a behavior, then match it to the incentives. With points as the foundation for gamification, such a feature would provide the reward. And Communities that tie these digital points into real-life benefits and rewards then have a stronger incentive to drive the behavior.