Generally I like the idea of generals seeing who is online and who is not ONLY at fort battles.
Yet, there are some technical issues, as I know from my IT/networking experience.
The game server does not know "naturally" when a user is online or offline.
Usually, a user who logged on is seen as ONLINE until he logs off.
Also, a user who logged on is seen as ONLINE until he closes the browser window.
But if there is a forced disconnection (power failure or connection lost for the user),
the server may keep seeing him as ONLINE.
So, OFFLINE status may be certain,
but ONLINE status is only probable...
Moreover, some users may stay ONLINE but not play the game and fight in the fort battle (if they have to watch TV, for instance...) just to prove to their generals that they are active - this would be an abuse.
So, the usefulness of this proposal is a bit tricky...