As regards the "sieges are ineffective against dwarves" discussion...
Part of the point of the Caravan Arc is that dwarven fortresses will not be able to be completely self sufficient. Your fortress will need to import other things, and your trading partners will presumably care enough about your exports to come to your aid. Personally, I think that as more complex social interactions amongst your dwarves are implemented sieges will get more interesting, with "siege mentalities," traitors, faction politics, and so forth being strained by the siege. The reason fortresses can be self sufficient right now is because the rest of the framework isn't there- a player can't reasonably expect to maintain a fortress if they must rely on trade. This is why the Caravan Arc is super exciting, if somewhat boring in its implementation.
Just about the entire point of the Army Arc is to make sieges themselves more interesting- make siege weapons work right, introduce enemies that will avoid walking through your trap hallway like cattle to the slaughter, adding new siege equipment/creatures that can destroy constructed (but not natural) walls, and of course the external political scene that sieges effect- the decisions of your allies, aid from the rest of your civ, sieges that can have endings other than total annihilation of one party or the other, etc. The reason this isn't in yet is... well, mostly it is just waiting on the Caravan Arc. As Toady has said, there is a lot of low-hanging fruit right now, so it just comes down to what bit of DF goodness gets added next.