Well, I know for myself: email is enough of a distraction without adding twitter to it. Rapportive provides a useful service by providing me more information about my contacts. By adding the ability to reply to twitter, though, it becomes more than service providing information; It ends up becoming another possible distraction. No, I don't have ADHD. I just have little self control when it comes to things that pull me out of writing. My brain resists work.

Also, of course, there's the fear of inevitable feature bloat. A service that works well at one thing should continue improving in that area without adding unnecessary features.