yeah, manikins aren't that deadly. but what makes em tough is high hp. stuff that'll clear skeletons has to hit 2 or 3 times to kill a manikin. and when you multiply that by, oh, 2000, it can be a real nightmare.
then you have all the songs/buffs that actually make manikins viable combatants.
believe me, C'tisian strategists have spent many sleepless nights staring across the ocean.