No, there's no plan to allow logins via user IDs. User IDs are not shown in the system, so unless we make it available, admins/clients wouldn't know what ID to login with. Regardless, logging in with a user ID can present ambiguity. For instance, if one person has a username of "2" and a different user has the user ID "2", which person is logging in? Usernames need to be unique to avoid this kind of issue.