I hardly doubt that someone could use a knife (or dagger) effectively on the open field, where the enemy can see you coming.
Maybe a knife (dagger) could be equipped onto a Champion as a "Secondary" weapon, enabling them to do sneak-attacks while in stealth mode ... but other than that I see no real use for Knives in combat.
I would consider this much more bothersome than the +6 initiative aspect, to be honest.
As I've said in another post, Wakizashis, Shortswords, and Gladii can be used for good *infighting*, but has anyone pondered the usefulness of a Tanto or Knife on the battlefield? These are for eating food, not for killing
I think the closest (in anything near reality) we could get to knives on some type of warrior would be a giant Meat Cleaver.