I will build a few forts early on at key strategic points and to avoid having to move men over difficult terrain, but by late game, I just take them over from others.
aka "We DO Not Sow".
By late game I actually find them a hindrance at times when for example I want my mages to be researching in a province, but also involved in defense. Having a fort makes that a problem and I have actually destroyed forts for that reason. Also, by late game, conventional troops are less relevant so I'd rather be spending my money on mages.