When already logged in, the login form (after I7e40c13a6ca5) will allow users to log in again as another user. However, in this scenario, LoginForm does not call $user->logout() on the original user.
The question: should it?
Extensions may be expecting the appropriate hooks to be called any time a user ends their session (and in this case, even though they are switching users, it can be considered an end to the session).
For example, Extension:SecureSessions keeps track of where a user is logged in, and shows that information to the user. However, it depends on the UserLogout hook to know when a session is terminated.