Personally, I prefer to discuss content on another site. Not because their commenting systems are better, but because I have a wide range of interests, and it is more convenient for me to discuss information where any content can come in.
Also, on a site dedicated to discussion, I don't have to worry about moderation or permission of the content creator.
Exactly. There is no point whatsoever in commenting on a multitude of blogs and website across the internet - then your opinions, views and contributions are fragmented in different places across the internet too; with no way to keep track of them, and no way to build rapport.
Also, on a site dedicated to discussion, I don't have to worry about moderation or permission of the content creator.