Yeah I love this one. Seen it a few times here and heard them called 'Dodge-me traps'.
Two viable routes for me so far:
1. You fit decent weapons and take a combination of strikes/dodges to thin numbers somewhat. in a large seige they will usually jam sooner or later and it prevents it getting overpowered, I like this because it lets my military play and means I dont just ignore seiges and get bored
2. You fit training weapons to minimize chances of kills/jams and use the fall to do the work.
Regarding the comment on installed spikes doing no damage beyond the fall itself. This seems fairly debated and i've seen claims with certainty for both sides. However, certain forum people whom I vastly respect for their encyclopedic knowledge of game mechanics have me assured that they DO deal additional damage. You use upright spears/spikes and dont lever link them, as I understand it this feature was actually added quite a long time ago and I believe one of the legacy versions of DF even included it in the combat logs, though 2010 does not.
My current favorite is the training spear route, like Bordellimies I also leave ramps back out of the pit leading to the outside, bad guys dodging off the first trap fall 1z and immediately go again, those making it a little further suffer various degrees of manglement and crawl their bleeding asses back for another go or pass out under your marksdwarf towers.
Its not necessary but its sadistic and I like it, sue me :p
Plus having no ramps into the pit means arranging a bridge/door/magma or some other viable method of retrieving corpses/goblinite