In 1.8, there will be a new event which triggers when the player logs in:
PLAYER_LOGIN
This will be sent immediately before PLAYER_ENTERING_WORLD when the player is logging in and when the UI is reloaded. It will not be sent when players enter/leave instances.
This can be used by addons that want to initialize data based on player state once the player is available, but don't need to do further work when crossing continent or instance boundaries.