If you keep your arms up like a boxer, the person wielding the knife could very simple turn it at an angle, and hit you in your upper arm.
And I have been taught knife defences in class, admittedly at a much slower speed and with fake knives, so no, I wouldn't claim myself to be an expert on blocking knives, which is what I meant by my lack of training. And how could you seriously say that an average guy could defend against a knife as well as a KM practitioner? Even with my limited experience in the class, I would feel more confident against a knife than I would before I had ever done those exercises in class. Why? Because I learned a new technique, went through it several times with a partner, and have therefore added that knowledge into my 'fighting portfolio'. I know that personally, if someone slashed at me with a knife before I started KM, I would likely flinch like a little ***** and take the blade to the chest, because I would have no idea how to stop it. Now having done what I did during those lessons, I would at least try to do what I learned in class.