Quick knife = good, as long as it does like 55% damage tops, so that you can't just shove your ass at a group of enemies and insta-kill away.
It must also be very "punishable", like a badly timed move in Street Fighter.
So, very quick draw and stab, then a significant delay before you can use your gun again.
It's supposed to be a last resort for the desperate, not a main weapon, I think.