I hear you, but I disagree with your presumption that you are unlikely to learn something from agreement. I think you are just as likely to learn from agreement as from disagreement, but most people just aren't as inclined to ask the question. Just to clarify one thing, I'm talking about thoughtful agreement and disagreement. "Me too" responses are not all that helpful, just as, "Nuh uh" are not helpful.
I think it's a just human nature. If someone says, "I like you," we don't generally ask questions. Hey, you like me. Awesome. When someone says, "I don't like you," a common response is, "What about me don't you like?" Surely, you wouldn't just dislike EVERYTHING about me. It has to be something specific... my (lack of) hair? My sense of humor? Something... but surely not everything.
Some advice I was given over 15 years ago when I first moved into management is to analyze my successes as vigorously as I analyze my failures. In general, most successful people do a good job of debriefing when things don't go well. We tend to analyze the situation, diagnose any failures and develop plans to address the areas where things went awry. When something succeeds, however, we tend to celebrate without analysis. It's human nature. We often just presume that everything went well because we're just that awesome. I came up with a plan. I implemented the plan and BLAMMO! Success!
My point is not to suggest that everyone needle everyone else whenever there is a click on a button. My point is that if you do it for disagreement, why not for agreement? My belief is that one is as valuable as the other. No more; no less.