Actually, I think the forts' ZOC would be a great addition IF the 50% bonus could be reduced or bypassed by aircraft. This would accurately reflect how forts worked in history. They controlled -- by virtue of their placement -- strategic positions. When placed, you'd typically try to put them in a location where you couldn't "just ignore them." You had to take them out or find another more difficult route.
One thing that's a problem, though, is that forts themselves often were NOT in land that you controlled specifically. It might be land you later INTENDED to place a city in (or a town/city would develop around a fort), but it wasn't necessarily within your territory in the sense of cultural borders.
If forts could be used as sort of advance locations OUTSIDE your borders (IE: romans building a fort in gaul before having actually conquered the land or built any towns), they'd be a lot more useful.
Currently, I use them VERY rarely if I have positions with choke points that lead to cities. Alternatively, I suppose they might be useful if you place them along your trade networks/roads to give you more ability to respond quickly to enemy mounted units. But as defensive positions, they're basically pointless -- the enemy can simply go around them at least 70% of the time.
Of course, over time, the practical effect of forts and static defenses is considerably limited
This basically culminates with the Maginot Line, however, when the notion of a fort or fortifications