I believe in the Quaker rule, "Only speak when you can improve the silence." Other people think speech is like squatting on land. You have to speak to gain footing. By charging people for the privilege of speaking, you make them consciously decide whether what they have to say is worth the $5 to join. They will probably say no.
I suspect that this is true, however. The earlier a comment is made, the more time it has to accrue upvotes before readers move on. I suspect there would be measurable correlation between the time of posting and the ultimate score of comments.