[SpiritWarrior]

Yup. This is why clerics are so adept against undead since many use bludgeoning weapons. Look at it this way: Poking a skeleton with a piercing weapon such as a spear will, most of the time do very little damage and half of the time do no damage at all since the spear would go right through the rib-cage or pierce already dead flesh. A slashing weapon such as a sword will also do little damage to bones, chopping and slicing small bits off maybe but nothing substantial.
A warhammer on the other hand crushes. Each blow can be potentially crippling and when were talking about a supernaturally animated creature, so delicate like the Skeleton, then bludgeoning weapons really get a chance to shine. Basically they can bash and shatter a skeleton easier than any other weapon type.

In my own server I have taken this a step further and actually added a damage vulnerability to skeletons and skeletal based creatures versus bludgeoning attacks. I have also added damage vulnerability versus fire to zombies, since I have always pereceived them having more 'dead-flesh' than skellys and thus easier set alight.