This is a silly argument. Example of a fort's defensive bonus sucking if it were lost;
A battle is taking place an an area, say 5 v 5 units... 1 of my units, the one occupying the fort perhaps, is killed, thus the enemy takes the tile... now there is a unit in the fort that is tougher to kill.
I'm not talking about holding grounds, the pros and cons of not advancing. I'm talking tile to tile combat mechanics. Losing a fort doesn't mean you lost the battle in that entire area, as you so implied. It means the unit inside the fort was killed and per the way the game functions, was occupied by an enemy unit.
So no... for the enemy to use the fort, it wouldn't mean halting his offensive. It would mean occupying the tile that it's on during a particular point where I'd like to attack him.
Sure, I could draw him out of the tile, but considering it's my fort, and thus in my territory, and I'm being assaulted... naturally, that would mean retreating deeper into my territory. Something I wouldn't exactly want to do. I'd rather not have built the fort in the first place.