iam going to design a system that has a login facility. After login the logged user can do several things so is it needs to include login usecase by that several usecases.
In most cases, it will only clutter up your diagrams. Login is neither complex nor does it give you much added value as a use case. In your textual document that includes the use cases with description, include as a prerequisite a list of features that require a successful login. Easier to track and to test.
来源：https://stackoverflow.com/questions/19443682/is-it-necessary-to-include-the-login-for-every-usecase