Yes, It would be nice to be able to do your own defense if you are on-line, but that isn't the way it is setup. One big issue is that you would be significantly slower than the AI, and that could be considered a significant detriment to the attacker.
What would be interesting, and perhaps possible, would be to be able to 'edit' the rules the AI used in defending your town. Tell it to hold back until is sees the whites of their eyes and then attack, or be smart and when the opponent attacks with 1 unit + 7 Rogues, kill the unit (THAT would make a big change to battles).