I personally consider doing anything that does not leave a 3-width path to my fort somewhat gamey (the idea being that's where the caravan would go, and I like to cover it, even though there is no caravan at the moment). Using bridges/etc to make my fortress unaccessible, in particular, I do not like. If I can raise the walls/etc and only have to worry about fliers... that takes the fun out of a siege.
I have a lot of traps, but because my dwarves have serious armor limitations (all armor grade metal is imported, since I only have galena). I want something that can handle large numbers, in a meat-grinder fashion - without needing cleaning. Hence I want a siege-engine based defense, and I have been hard at work producing masterwork siege parts and training my operators (3 legendary operators now).
Chewie's idea, albeit with 3 tiles wide, is one I'll work with... accuracy is still a serious limitation, as the do have a tendency to fly 15-30 degrees wide. For instance with the illustrated layout, I'd be losing about 1/3 of the arrows of the top and bottom weapons. Well - assuming the open space at the end is enough that goblins in the fortification maze do not cause operators of the engines to flee.
Siege weapons are also, in my experience, the best way to deal with a troop of goblins with a captured leader, especially ranged ones. Attacking a group of 20 goblins standing around their captured leader's cage is a recipe for a lot of fatalities; there's just no way to get your dwarves to attack in a coordinated-enough fashion for it to be otherwise. One or two will lead the charge and take several volleys of arrows before the main group starts to engage them in close combat. It also works the same with human sieges, which tend to mill around for a while.
I tend to wind up with milling-about goblins, usually because their leaders trip the early-warning cage traps. In one brilliant effort they turned on the skinless administrator demon that came with them, but they could not really hurt him with their silver arrows, and wound up chasing him all over the map, resulting in more than a dozen in cages. Now leaderless, the remainder milled about aimlessly. I don't want casualties (and rag-tag-armored dwarves will have them), so what else can I do?