Wouldnt even bother trying to defend against a knife or take one. Im currently dealing with the repercussions of a cut from in dojo training where I knew the attack was coming, what attack it was and I still f*ucked it up, the end result isnt something I want to experiance again in whatever shape or form.
Maybe if I also had a knife I might begin to think about facing an aggressor with a knife, but I would expect to get cut. Its how little you get cut and where you receive these cuts that will determine your chance of survival. (But saying as im not in the habit of carrying a knife then this is a moot point