The most powerful mundane piercing weapon I ever owned was a dagger, so the "power level difference" is entirely debatable. In fact, that dagger was only marginally less effective than some two-handed swords.
Of course, it was a dagger I got off a gypsy
. But...there are plenty of halfswords that have high damage crits. They just don't seem that way very often because most warriors either roll with slashing or chopping or bludgeoning, because of the regional starting bonuses or stun damage, and most "piercey" guilds just suck total balls forever and never get to the point where they're rocking the head and neckshots.
On topic for the backstab element: there aren't many chopping or slashing weapons that are 5 stones or less that are so out-of-range powerful that they'd be unbalanced. I'd say if you put them stone-for-stone against similar halfswords, they would generally be less powerful, honestly.
On topic for the poisoning element: it's kind of gamey to say "oh, well, the downside of using poison is that you have to train your favorite weapon -and- train piercing in order to deliver the poison," when there isn't any other good reason.