Well I've asked this EXACT question before but have since forgotten the answer... will remember to write it down this time... I've got this quote recorded in a file... can't remember where it's from though...
"First, anyone who grabs a dagger to injure a man he knows to be unarmed will strike a blow in the first opening he fancies, since he will only be concerned about his victim’s fleeing before receiving his wound. Therefore, this type of assailant will operate without subtlety. And for this reason, the man being assaulted can defend more easily."
The Iron Door Collective
Your meticulously researched 15th century longsword techniques cannot harm me- my wings are like a SHIELD OF STEEL!