If you don't know how to use a knife, pulling it in a dangerous situation could result in the knife being used against you, with fearsome results. If you are even thinking of using a blade in self-defence, take courses so you know what you are doing.
Actually, any determined person armed with a knife is dangerous. It doesn't matter if someone has 30 years of martial arts experience, is a 5th-degree black belt, is a collegiate wrestler or a bodybuilder. An untrained but determined 13-year-old kid armed with a blade will have the advantage. Now imagine that kid trapped in a corner; good luck trying to disarm him and still emerge unscathed.
Not to say that training to use a blade isn't worthwhile, anything that can help you certainly is. But people are killed by blades weilded by untrained people all the time.
About 20 years ago, I knew a guy who was at the time a 3rd-degree MA instructor. He handed me one of those hard-rubber self-defense training knives and asked me to try and "cut" him with it so he could practice his 'knife defenses'. He would also try to disarm me. The only rules were that I was to act like I really wanted to get him/avoid him and not cooperate with him; and for safety, I was not to attack his neck/face (he wasn't wearing any eye/face protection). I had over a decade of MA training of my own at the time, but no knife training at all, and he was larger than me and had about the same speed. So basically I wasn't overly different from an untrained person with a knife.
Well, in the space of about 5 or more minutes, I "killed" him several times. I had stabbed him many times and gave him countless slashes. He ended up with bumps and scratches and some slight cuts all over his arms, chest, torso, and backs of his hands, because though rubber, that knife was stiff and not floppy. In fact, it wasn't difficult at all to 'cut' him, but he was never able to effectively get his hands on me, strike or kick at me without getting 'cut'.
Of course, this wasn't the same thing as a random attack and self-defense, but it does show that any determined knife-wielding person is most definitely not easy to disarm. I would imagine if he'd asked to do the drill with someone else it wouldn't have turned out much different.
It's always a great idea to be able to run away, but that isn't always an option. Besides the earlier-mentioned scenario where a wife or child is with you, also consider if a person is much older, or has some injury or physical condition that prevents him/her from outrunning the attacker(s). Many young criminals are in far better physical condition than those they prey on.
Jim