To be frank, people using IM to communicate with me disrupt my flow a lot more. They start a thread/conversation/request, and then are slow to respond, or disappear. You can't do that face to face.
Face to face (or even telephone) forces people to prepare. Rarely do I see people work that way with IM. People tend to realize that they are interrupting someone if it's a face to face. In my experience, many (most?) workers don't view IM as a significant interruption to the other party. So they feel much more free in using it.