And as far as realism is concerned, they usual piercing/cutting/blunt categories also don't actually make too much sense.
Take a mail armor for example, the usual paradigm is that it stops slashing attacks well, piercing mediocre, and does badly against blunt weapons. But looking at piercing weapons, it's a simple consideration whether the piercing weapon is small enough to go through a ring or not, otherwise it's as good as blunt. And a cutting weapon is as good as a blunt weapon. Then it's just a matter of how much force your blow.
Actually, you just need to force the tip of your piercing weapon into a ring, then depending on force you may succeed at breaking the links and penetrating or not.
Anyway, to address cutting/piercing/slashing:
They make whole lot of sense because they describe different approaches to dealing damage. Both cutting and piercing focus on putting their momentum in smallest possible area - point or line - in order to penetrate and disable target.
If they fail to penetrate, they do indeed turn into blunt weapons (they even do that when they succeed, although it might be lesser concern in that case and if they go all the way through they only inflict a fraction of blunt force trauma they could), the thing is that actual blunt weapons (spikes and flanges aside) are built with an assumption of failed or limited penetration, so they attempt to really make best of the blunt force trauma and deliver as much momentum and kinetic energy to the target as possible.
Ideally system should work as follows:
Weapon tries to penetrate and inflict some localized critical damage according to the depth of penetration. If successful, weapon may require special pulling out move and may get stuck, depending on the depth of penetration unless it was a slashing attack and weapon penetrated fully (meaning it either lopped off a bodypart or exited after partial cut).
Blunt weapon will either fail to penetrate or penetrate shallowly, making pulling it out non-issue.
A weapon's kinetic energy/momentum used up during (attempted) penetration is then used against target as blunt force trauma with it's own DT/DR formula.
It might make sense to split cutting into hacking and true cutting (curved. swords.), with latter being especially shitty at energy/momentum transfer being both blessing (slicing through soft targets like a pro, without stopping, inflicting massive injuries with severe bleeding and good chance of dismemberment or slashing something important, and generally ignoring padding due to not wasting energy on blunt force trauma), and curse (trying to cut through plate or mail would be just lol).
So:
-blunt dagger would be only marginally better blunt weapon than unarmed attack by non-martial-artist, and wouldn't break through golem's DT.
-soft targets that don't care about bleeding or criticals against organs (like undead) would be fucking scary to most piercing/cutting damage users because typical sequence would involve having weapon get stuck in the target or overpenetrate on thrust, target not particularly minding it and proceeding to nom wielder's face.
Your best bet would be either bludgeoning them, which might not be particularly effective, but at least doesn't create risk of having your weapon stuck in unfazed target, or using some really good true cutting weapon, that can be counted to enter *and* exit, and just lopping them to pieces to ensure their incapacitation.
. Anyway, more info here: 
