While I think this idea has merit, I like the fact that only members who have met certain criteria can vote on debates. I propose the following:
Tie an opinion-like section to every debate, or allow the instigator of a debate to add one to it. That way, even if no one votes, there can still be a robust discussion on the matter, with the percentages of people who feel one way or another. This would be separate from and in addition to the current voting system.
I use debate.Org. Many other people do, too. These people share this website. More and more people use it because of the people that use it.That points the whole thing out this is popular and people are using it. If more people are using it, then it is obvious that people are going to vote.