Zuvembies, ascomoids, and ningens all have DR/Piercing. It is a rarer type of DR, but there are some monsters that have it.
Nosferatu have DR/wood and piercing. Not piercing by itself, but it is a subset of DR/piercing.
Rakshasa have DR 15/good and piercing (& variants introduced later have various other DR which includes piercing). More importantly piercing weapons are at a relative advantage underwater, if you don't have a freedom of movement effect.
Yeah, Rakshasa are pretty much the only thing even near common (because they are teh evil outsiders to use in any vudrani campaigns, and generally easy to understand- animal headed evil outsiders). That, and maybe some spells that try to take advantage of the fact that most players don't go for piercing much because it doesn't give as much of a benefit.
Really, I suspect that piercing was originally a grandfathered nerf from the old D&D stuff. Look at what has piercing:
-A lot of simple weapons (which are supposed to be 'worse' than martial weapons). Notably, there are no one or two handed slashing weapons that are simple. Note that simple still has bludgeoning, because removing that would cripple the melee person against the EXTREMELY common skeleton and such.
--notably, the best str based simple weapon, the long spear, is piercing. The only reach weapon available to simple weapon users.
- Archery of pretty much all sorts. I know we have bludgeoning arrows NOW, but this is about what was used when the three damage types were originally made. And I think we can all agree- there are plenty of reasons to nerfing archery.
-rapiers. The obvious pick for a dex user, since it was high crit. The only high crit weapon given to rogues.
From all this, you can begin to see that piercing may have been meant more as a punishment for stuff that was 'too good' for its purpose otherwise. Of course you rebel against the idea of nerfing these things... but we are talking about the sensibilities when an earlier D&D edition was created- a period where things were a loooot more small scale (heck, you might be considered good for a str build if you had 12 str back then).
From all this, you can begin to see that piercing may have been meant more as a punishment for stuff that was 'too good' for its purpose otherwise.
I suspect the reasons are mostly thematic rather than game-balance related. Piercing weapons are piercing because they're pointy. Simple weapons are simple because they sound like they ought to be simple. DR/Piercing creatures are that way because it fits with their legends. Nosferatus are based on vampires that you're supposed to pierce with a stake. Rakshasas in the Indian legends were often defeated by heroic archers. Etc.
From all this, you can begin to see that piercing may have been meant more as a punishment for stuff that was 'too good' for its purpose otherwise.I suspect the reasons are mostly thematic rather than game-balance related. Piercing weapons are piercing because they're pointy. Simple weapons are simple because they sound like they ought to be simple. DR/Piercing creatures are that way because it fits with their legends. Nosferatus are based on vampires that you're supposed to pierce with a stake. Rakshasas in the Indian legends were often defeated by heroic archers. Etc.
I do not mean that piercing weapons shouldn't be in the simple category....
....but why are there no simple slashing weapons that can be 2 handed. There are light ones- daggers and sickles, etc. But nothing that can be two handed, not even a crude cleaver with mace like stats. Or maybe an simple, small-ish ax (such as the battleaxe).
That is the telling part. Combined with the fact that piercing is on bows (something that you definitely want to nerf), and most of the marital weapons that get passed to classes that don't get full martial weapons (short swords are on most of the older classes that melee, even monks, and rogues get rapiers), and it starts to paint a picture.
Gasburst Zombie (though I think it should be DR -Piercing or Slashing instead..)