Well, if there is an internal battery, it'll "kong" sooner or later, and since we can't open it and replace it ourselves, there will be more warranty claims, more complains, more jobs and more hassles to M$ support. If the battery is located externally and reachable by users without voiding warranty, you know, knowing how intelligent people can be, it means trouble also. So it is a lose-lose situation to include a battery just for the sake of time.