Forts never figure much into my style of play. As it is by I usually the one on the offense in a war. The couple of times i built a fort the enemy just by pass it and i usually by pass the fort as well when invading.
I don't use forts a great deal either but when I do use them they're more often for an offensive position or advantage than defensive. Obviously it makes sense for your enemy to go around your forts, but you can use that to your advantage. It is where you can base some of your reactionary troops and extra aircraft etc. Paradropping from them and basing missiles are offensive advantages.