I can confirm this. And to add to it: this
might have something to do with claimed territory. AI "wastes" a huge proportion of its units on such exclaves. A stack of doom on each claimed tile. Sometimes you'd say, fine, that iron resource is indeed that important. Other times it's just obsidian, a resource the civ already has access to otherwise, or even no resource at all (maybe a future one to still pop up?).
The result is just what you've observed: cities of AI are barely defended. In my latest game, the mighty Aztec lost a bunch of "important" border-cities to some inferior Gilgamesh - only because all of Monti's units were guarding such exclaves, instead of the cities (or mind you; using them offensively/to attack instead...). It was terrible to watch.
Maybe some weighting/modifier (or what not) is a bit off. Can't tell.
Another thing which currently drives me crazy is the fact that the two cities of my first colony (somewhere overseas) is in a constant state of revolution, to the point where you can't do anything. One city was a single-tile island, resulting in a lost/barbarian city every friggin time a revolution occured (each few turns...). Which was super strange, so I've opened up the world editor to make a two-tile island out of it. Now I could at least keep the city in my hands most of the time, probably due to the fact that the revolutionary units could spawn somewhere.. (this is really super annoying).
But despite being one of the leading powers (science, culture, military), those two cities kept revolting. So I've put spies there. Turns out that virtually each turn some barbarian spy spawns out of nowhere (confirmed by catching them every few turns and opening the world editor to see barbarian spies?!) to force those revolutions turn for turn... That's just nasty/cheap.
I love the revolutions mod - IMHO the best thing that ever happened to civ - but this drives me crazy.
There is something completly wrong with barbarian spies.
