What's the point in posting something without discussing it?
That's a very good point. I can think of at least one time in which I've posted a link without posting any sort of comment on it. But in those instances, I think the article speaks for itself. Regardless, writing some sort of thought on the matter I would argue would facilitate more discussion.
Sometimes I don't know what I want to say about a link, and just throw it out there, hoping for feedback. I think many people assume that the poster is endorsing the content of any given link. However, sometimes I post things that I disagree with, just because the perspective interests me.
And that's awesome! That controversial topic will get people talking and perhaps have someone think of something they've never thought about. Through discussion on a few threads that's happened to me often. It's an incredible feeling to have thought a new thought and it's an experience I'd love for others to have.