Offline Gmail certainly has an Outbox. The main reason being you'll generally have no internet connection or a very flaky one. By product of it is that yu can indeed cancel emails after sending them.
It wouldn't be a bad idea to use an Outbox for some people. Gmail could offer it as a labs feature and whenever you request or end your session/logout it would send your email...
Removing email from an account once sent like above seems like a very bad user experience quirk.
