I suggested this a long time ago, to no avail.
I think it would be very awesome, (perfect for Ninjas) and realistic. A slow, light, only thrust weapon, made only for backstabbing, that can't block anything.
Realistic because if you were sneaking up on someone you should be able to find a crack in someones armor and stick your dagger straight through the neck or balls or somewhere.. xD
I also bet someone would find other creative uses for it, and killing with it in a serious fight would be somewhat a prestige kill.
Make it happen! :-) Urist? Why not? :-)