Blog comments generate a community around a blog. Everyone who visits the blog is able to read its comments. Most of your visitors come to your website from search engines (think Google) and generally they are not interested in reading the comments.

If a user comments on a post FriendFeed, the content appears on the stream of people who follow him. That content might potentially interest his friends, particularly because he liked the article enough to post a comment.

If friends of your friends also comment on the post, then even more people will see your content.
To see new comments, the user generally has to visit the blog, and sometimes they forget your blog address. If your comments are powered by FriendFeed, staying updated with the discussion means visiting the social network and not having to remember the blog address each time.