From reading the article, it seems that the BBM integration has to do with setting your BBM status to what you last tweeted and accessing buddies Twiter accounts directly from their BBM account. BBM is more of a social app while iMessage is just a another form of communication, like SMS 2.0. Apple would need to enhance iMessage's features to allow this type of integration. *cough* iChat *cough*
The integration is kinda nice actually after having used it myself and I think it would be good to see some of the features on iOS as well.
One feature is to update your BBM status, of course that is useless on iOS currently.
The other great thing is you can click a tweet and share it right with a contact, it sends them the tweet in a message, or you can send them a link. Makes it super easy and fast. I believe on most iOS twitter clients you can share via email, but not iMessage (though could be 100% wrong on that)