FriendFeed provides one-stop-shopping for viewing and commenting on your friends' activities; convenient and easy.
Facebook requires you to navigate to each friend's Mini-Feed to add comments; NOT convenient and NOT easy.
Adding comments to the main News Feed will help, as long as they also include any comments from friends to my news item in the same feed AND they include any items that I have chosen to comment on in the same feed.
I want one-stop-shopping for all of this back and forth commentary.
I also agree with Manogr on having a comments area that enables you to access historical commentary (ie. comments no longer visible in main feed).