I've thought of doing that. Maybe. But not all comments disagree, and frankly I wouldn't want to do anything that increased the proportion that do.
"And when you agree there's less to say. You could expand on something the author said, but he has probably already explored the most interesting implications. When you disagree you're entering territory he may not have explored"
To me, cooperative, exploratory statements are the most valuable.
"That is a interesting opinion, I've never heard that perspective. But what about case xyz, have you considered that."
In that context, it is much easier to build up interesting information trees, rather than simply staying stuck on one (often minor or irrelevant) node.