I think the idea behind a fixed battery is that it would outlive the life cycle of the computer. At least from a computing standpoint, the life cycle of computers is usually 3-4 years. Granted, being able to switch batteries may be important for those who travel and need extended battery life.
Edit: The one thing I wish they did for the entire line is add the anti-glare option for the screen. I have a 15" and wish it was available as an option when I purchased mine. As it stands, the option is only available on the 17". (Correct me if I'm wrong here)