For me personally the usability versus the technical implementation is not worth. It would be good probably for the topics in whole, but for separate posts?.. What sense does it make?
It also reminds me the "karma" add-on, where you could vote for the post, but increasing / decreasing the amount of user's "virtual karma". There should be a lot of limits, for example, you can't give more karma as you have, and only for a limited number of people per 24 hours.
However I don't see the usability of such add-on as well.