Fighting this killer feature is Microsoft Word's own killer feature, which is: Everyone in business has Word, and most people know how to use it effectively. There are plenty of people who would use a simultaneous editing feature in Word if it had one, and who aren't going to switch to Google just because it does.
A new service, Plutext, currently being developed, will bring nearly live editing to Word documents. I saw a demo at the Office 2.0 conference.