Agreed 100%. I also had a siege recently. The problem wasn't the 133 human soldiers who invaded, or my near-total lack of traps, 'cause they only brought 4 warlords with them, had almost no ranged weapons, and let me raid them a couple of times around their campfires before rushing the gate.
No, it was the combination of:
1) The failure of my military dwarves to set up any kind of synchronized sleep/eat/drink schedule within squads, or to resist bodily urges in a time of peril even for just a little bit, obliging me to mobilize 'em all and wait for those who felt like it to show up, then form temporary squads and use them quickly before they scampered off. The human squads, in contrast, all moved in unison...
2) The extreme difficulty of kitting up with appropriate weapons and armor as opposed to the silver weapons I was using for sparring and the bronze/leatherwear that it's a PITA to get them to stop putting on again. This was a big reason for not having too many melee chaps other than wrestlers, and for having no speardwarves at all, 'cause my barracks wouldn't just be covered in blood then (flowing out the door even!), it'd be decorated with limbs and spleens.
3) The insistence of my dwarves at charging the enemy with totally inappropriate weapons (unloaded crossbows) and attacking regardless of numbers and disposition, despite setting them to "stay near station", setting the station directly away from the enemy, and even ordering all dwarves inside. The only tool I found that worked on a suicide charge was deactivation - turning them into unarmed, unarmored civilians. And that desperate move cost me half my total deaths.
4) The entire nonchalance of my civilians, who (as pointed out before) behave exactly the same in time of danger as they do when the coast is clear, except for milling around near the entrance if you turn on "Dwarves stay inside".
++++++
Had the humans simply rushed the gate, I estimate that 5-10 civilians would have been milling around, half of my military would have been away, and many of those who did show would be using inappropriate gear. Can we say "massacre-in-detail"? It's no wonder people use traps so much!
The only thing that saved me was machine-bows. Two or three marksdwarves (out of 12) would show up at the assembly point at approximately the same time, I'd send them off raiding the furthest campfire from the gate, they'd spray death on those who deserved it, and then I'd turn them into civilians so that they would run away when out of ammo. Killed 30 to 3 that way.
I then tried using melee dwarves (wrestlers mostly, plus a trio of well-trained mace/axe dwarves). Enough showed up at the assembly point at one time that I zerg-rushed the nearest campfire to the gate. Big mistake. Oh, sure, the warlord-less human squad around that campfire got massacred with no dwarven loss, but the remaining humans finally charged the gate, and we were in the way. Seeing some of my melee chaps mosey off for tea, and some wandering about, and some charging into the oncoming warlords without backup, I decided to deactivate them all and flee. The humans killed 5 before we got inside, despite the intervention of some marksdwarves, who slew enough to break the seige.
++++++
Oh yeah, and one other thing: This whole war - which may easily cost hundreds of lives before it's over, to say nothing of years of lost exchange - was caused by a human Trade Baron getting stomped on by an elephant appearing right at the edge of the map. Not only are my dwarves innocent of murder, they weren't even allowed to patrol out far enough to have any chance at stopping it.
... this is why I'm so happy to hear that military stuff will be redone at some future point. That'll be a happy day!