Back when the goal was for score, the siege units had a huge purpose. But the removal of score (which was a super good move in general) has caused some need for rethinking some things (like this).
I'm thinking that maybe worrying about their balance might be best left till after cataclysms are in?
I mean, ok, they're not useful now, but the reason is simple: There's no reason to take down cities, because each side most of the time has the same number.
When cataclysms start.... this is likely to change. If the reds have 7 cities, and the blues end up with like 4 (as a result of an event), then you bet I'll be pumping out a bunch of siege units in an attempt to fix that number, particularly because BUILDING an entirely new city is not always a viable option, depending on the map and what's going on.
This is how it seems to me. Siege units actually do their jobs very well; there simply needs to be reasons for them to do it.
Also, I agree with Zharmad above me, I think.... those funky inhabited forest buildings, I dont even know what those DO. I keep sticking them into maps to see if they eventually do something, they keep not doing something. The idea of a bandit stronghold tile showing up at times would be a good one, I think! It could have a cooldown like military buildings do, and pump out a bandit every couple of turns. Siege units would be good here indeed. It'd be a nice use for the commandment thing as well (though I'm thinking THAT too will have much more uses when cataclysms are in)
Also, the Dead Forests..... how do they work? They dont seem to actually spawn bandits ON them, so I'm a bit confuzzled on their function.
Still, I love the thought of a bandit stronghold like that, a proper building that they can have.