First, some people don't want an opinion different than their own. I accept that and either alter my post or don't post based on that. In real life, some people aren't going to change their mind...no matter what you say, I usually reach a point where I don't feel like wasting my time to get them to see they aren't right. (This is usually in regards to people feeling worthless, etc.) I fight people's low self-esteem until I feel they are no longer listening....then I agree to disagree.

As far as helpful or insightful. It's usually people suggesting ideas I haven't thought of or other points of view.

Hope that helps!