I got together with a few other guys that trained manny different things (from BJJ to JJJ to Kali to Systema to Nintendo-ryu), and what we did was have one guy with a rubber knife, the other one unnarmed - and running not being an option.
Headgear, shin guards, groin protectors (and mouthguards, of course).
What i got from those "sparring" sessions is this:
Bottom line:
1.- If you're against someone who doesn't know how to use a knife, you're screwed. (most of us got "killed" 8 out of 10 times against these).
2.- If you're against someone who does know how to use the knife, you're utterly screwed. (against the kali, systema, or jjj guys, it was more like 9/10 - and the one time the unnarmed guy made it out, was because the guy with the knife screwed up majorly).
3.- If you're against someone who knows how to use the knife, and also is "better" in unnarmed combat, you're totally pwnt - because if you had already little to no chance to beat him while he's unnarmed, your chances go near 1% when he is armed with a knife - he will kick, punch, headbutt, slam, throw, sweep, bite, and choke you... while he stabs you.
4.- Nintendo ryu guys were the least effective with the knife, but mostly because of the fact they were intimidated to some extent by the "trained" guys (this is just my opinion). Which doesn't matter that much, because in the street, "Mr. Juan with a knife" doesn't know you are a black belt in Judo and just won a medal in the Combat Sambo European Championship, so he'll attack you just the same.
5.- If there's a knife, there's going to be a cut. Getting cut doesn't mean you have to stop defending yourself - use your hands! It's better to get your hands sliced over and over again, than having your neck sliced one time.
6.- While some styles proved good to avoid/stall getting cut, they were entirely unable to disarm or defeat the armed attacker - so if your mom is not coming to help you, you're screwed by being passive.
7.- It was just a very hard sparring session with protective gear and a rubber knife, so 95% of this is entirely innacurate and should not be given too much thought
