The chat threading while typing SMS is awful.The Screen is full of unnecessary Information instead of showing the SMS text. I've read, that in former times there was no possibility to get rid of the chat screen. (Only to change back to BB 8300!!, look on the right side of this editing screen) Exists meanwhile a way to edit SMS in a comfortable way without chat.
Please recognize, that the handy is used by many people as an instrument of professional contact. There is no room for flat language or wrong spelling. These people want to see and control, what they will send to their corresponding partners. With your chat screen, it is send blind.